Remove most traces of WinHelp support.

Remove the 'winhelp-topic' IDs from the Halibut source, and from the
code. Now we have one fewer name to think of every time we add a
setting.

I've left the HELPCTX system in place, with the vague notion that it
might be a useful layer of indirection for some future help system on a
platform like Mac OS X.

(I've left the putty.hlp target in doc/Makefile, if nothing else because
this is a convenient test case for Halibut's WinHelp support. But the
resulting help file will no longer support context help.)

doc/pubkey.but

@@ -61,8 +61,6 @@ The key types supported by PuTTY are described in \k{puttygen-keytype}.
 
 \H{pubkey-puttygen} Using \i{PuTTYgen}, the PuTTY key generator
 
-\cfg{winhelp-topic}{puttygen.general}
-
 PuTTYgen is a key generator. It \I{generating keys}generates pairs of
 public and private keys to be used with PuTTY, PSCP, and Plink, as well
 as the PuTTY authentication agent, Pageant (see \k{pageant}).  PuTTYgen
@@ -106,8 +104,6 @@ server to accept it.
 
 \S{puttygen-keytype} Selecting the type of key
 
-\cfg{winhelp-topic}{puttygen.keytype}
-
 Before generating a key pair using PuTTYgen, you need to select
 which type of key you need. PuTTYgen currently supports these types
 of key:
@@ -133,8 +129,6 @@ supported by PuTTY are RSA, DSA, ECDSA, and Ed25519.
 
 \S{puttygen-strength} Selecting the size (strength) of the key
 
-\cfg{winhelp-topic}{puttygen.bits}
-
 The \q{Number of bits} input box allows you to choose the strength
 of the key PuTTYgen will generate.
 
@@ -149,8 +143,6 @@ equivalent security to RSA with smaller key sizes.)
 
 \S{puttygen-generate} The \q{Generate} button
 
-\cfg{winhelp-topic}{puttygen.generate}
-
 Once you have chosen the type of key you want, and the strength of
 the key, press the \q{Generate} button and PuTTYgen will begin the
 process of actually generating the key.
@@ -177,8 +169,6 @@ appear in the window to indicate this.
 
 \S{puttygen-fingerprint} The \q{\ii{Key fingerprint}} box
 
-\cfg{winhelp-topic}{puttygen.fingerprint}
-
 The \q{Key fingerprint} box shows you a fingerprint value for the
 generated key. This is derived cryptographically from the \e{public}
 key value, so it doesn't need to be kept secret; it is supposed to
@@ -193,8 +183,6 @@ utility, will list key fingerprints rather than the whole public key.
 
 \S{puttygen-comment} Setting a comment for your key
 
-\cfg{winhelp-topic}{puttygen.comment}
-
 If you have more than one key and use them for different purposes,
 you don't need to memorise the key fingerprints in order to tell
 them apart. PuTTYgen allows you to enter a \e{comment} for your key,
@@ -213,8 +201,6 @@ PuTTYgen, change the comment, and save it again.
 
 \S{puttygen-passphrase} Setting a \i{passphrase} for your key
 
-\cfg{winhelp-topic}{puttygen.passphrase}
-
 The \q{Key passphrase} and \q{Confirm passphrase} boxes allow you to
 choose a passphrase for your key. The passphrase will be used to
 \i{encrypt} the key on disk, so you will not be able to use the key
@@ -258,8 +244,6 @@ a result.
 
 \S{puttygen-savepriv} Saving your private key to a disk file
 
-\cfg{winhelp-topic}{puttygen.savepriv}
-
 Once you have generated a key, set a comment field and set a
 passphrase, you are ready to save your private key to disk.
 
@@ -274,8 +258,6 @@ will need to tell PuTTY to use for authentication (see
 
 \S{puttygen-savepub} Saving your public key to a disk file
 
-\cfg{winhelp-topic}{puttygen.savepub}
-
 RFC 4716 specifies a \I{SSH-2 public key format}standard format for
 storing SSH-2 public keys on disk. Some SSH servers (such as
 \i\cw{ssh.com}'s) require a public key in this format in order to accept
@@ -300,8 +282,6 @@ public keys.
 \S{puttygen-pastekey} \q{Public key for pasting into \i{authorized_keys
 file}}
 
-\cfg{winhelp-topic}{puttygen.pastekey}
-
 All SSH-1 servers require your public key to be given to it in a
 one-line format before it will accept authentication with your
 private key. The \i{OpenSSH} server also requires this for SSH-2.
@@ -317,8 +297,6 @@ public-key authentication once you have generated a key.
 
 \S{puttygen-load} Reloading a private key
 
-\cfg{winhelp-topic}{puttygen.load}
-
 PuTTYgen allows you to load an existing private key file into
 memory. If you do this, you can then change the passphrase and
 comment before saving it again; you can also make extra copies of
@@ -337,8 +315,6 @@ for information about importing foreign key formats.
 
 \S{puttygen-conversions} Dealing with private keys in other formats
 
-\cfg{winhelp-topic}{puttygen.conversions}
-
 Most SSH-1 clients use a standard format for storing private keys on
 disk. PuTTY uses this format as well; so if you have generated an
 SSH-1 private key using OpenSSH or \cw{ssh.com}'s client, you can use