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 07af4ed1 due to a branch management error and hastily
removed again in 29e8c24f. This time it really does match reality.)

[originally from svn r9998]

[originally from svn r9365]

[originally from svn r9202]

[originally from svn r7488]

[originally from svn r7146]

[originally from svn r5598]

tag, but that's OK because SVN doesn't distinguish tags from
branches anyway...)

[originally from svn r5361]

[originally from svn r4699]

[originally from svn r4430]

[originally from svn r3820]

[originally from svn r2210]

the latest release. This is so that .tar.gz snapshots for the Unix
port can be versioned as `0.53-20021016' or similar, meaning that
(e.g.) Debian version numbering can be monotonic between releases
and snapshots.

[originally from svn r2083]