Update version number for 0.68 release.

This commit also updates the dumps of Plink's and PSCP's help output,
adding the -proxycmd option to both and the -shareexists option to
Plink.

(Or rather, _re_-adding the latter, since it was introduced in error
by commit 07af4ed10 due to a branch management error and hastily
removed again in 29e8c24f9. This time it really does match reality.)

doc/plink.but

@@ -41,7 +41,7 @@ use Plink:
 
 \c Z:\sysosd>plink
 \c Plink: command-line connection utility
-\c Release 0.67
+\c Release 0.68
 \c Usage: plink [options] [user@]host [command]
 \c        ("host" can also be a PuTTY saved session name)
 \c Options:
@@ -54,6 +54,8 @@ use Plink:
 \c   -P port   connect to specified port
 \c   -l user   connect with specified username
 \c   -batch    disable all interactive prompts
+\c   -proxycmd command
+\c             use 'command' as local proxy
 \c   -sercfg configuration-string (e.g. 19200,8,n,1,X)
 \c             Specify the serial configuration (serial only)
 \c The following options only apply to SSH connections:
@@ -83,6 +85,8 @@ use Plink:
 \c   -sshlog file
 \c   -sshrawlog file
 \c             log protocol details to a file
+\c   -shareexists
+\c             test whether a connection-sharing upstream exists
 
 Once this works, you are ready to use Plink.
 

doc/pscp.but

@@ -39,7 +39,7 @@ use PSCP:
 
 \c Z:\owendadmin>pscp
 \c PuTTY Secure Copy client
-\c Release 0.67
+\c Release 0.68
 \c Usage: pscp [options] [user@]host:source target
 \c        pscp [options] source [source...] [user@]host:target
 \c        pscp [options] -ls [user@]host:filespec
@@ -63,6 +63,8 @@ use PSCP:
 \c   -hostkey aa:bb:cc:...
 \c             manually specify a host key (may be repeated)
 \c   -batch    disable all interactive prompts
+\c   -proxycmd command
+\c             use 'command' as local proxy
 \c   -unsafe   allow server-side wildcards (DANGEROUS)
 \c   -sftp     force use of SFTP protocol
 \c   -scp      force use of SCP protocol