Unix Pageant: implement GUI passphrase prompting.

I've written my own analogue of OpenSSH's ssh-askpass. At the moment,
it's contained inside Pageant proper, though it could easily be
compiled into a standalone binary as well or instead.

Unlike OpenSSH's version, I don't use a GTK edit box; instead I just
process key events myself and append them to a buffer. The big
advantage of doing this is that I can arrange for ^W and ^U to
function as they do in terminal line editing, i.e. delete a word or
delete the whole line.

^W in particular is really valuable when typing a multiple-word
passphrase unseen. If you feel yourself making the kind of typo in
which you're not sure if you pressed six keys or just five, you can
hit ^W and restart just that word, without either having to go right
back to the beginning or carry on and see if you feel lucky.

A delete-word function would of course be an information leak in even
an obscured edit box (displaying a blob per character), so instead I
give a visual acknowledgment of keypresses by a more ad-hoc means: I
display three lights in the box, and every meaningful keypress turns
off the currently active one and instead turns on a randomly selected
one of the others. (So the lit light doesn't even indicate _mod 3_ how
many keys have been pressed.)

Recipe

@@ -303,9 +303,9 @@ puttygen : [U] cmdgen sshrsag sshdssg sshprime sshdes sshbn sshmd5 version
 pscp     : [U] pscp uxsftp uxcons UXSSH BE_SSH SFTP wildcard UXMISC
 psftp    : [U] psftp uxsftp uxcons UXSSH BE_SSH SFTP wildcard UXMISC
 
-pageant  : [U] uxpgnt uxagentc pageant sshrsa sshpubk sshdes sshbn sshmd5
+pageant  : [X] uxpgnt uxagentc pageant sshrsa sshpubk sshdes sshbn sshmd5
 	 + version tree234 misc sshaes sshsha sshdss sshsh256 sshsh512 sshecc
-	 + conf uxsignal nocproxy nogss be_none x11fwd ux_x11 uxcons
+	 + conf uxsignal nocproxy nogss be_none x11fwd ux_x11 uxcons gtkask
 	 + UXMISC LIBS
 
 PuTTY    : [MX] osxmain OSXTERM OSXMISC CHARSET U_BE_ALL NONSSH UXSSH