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
(Or rather, _re_-adding the latter, since it was introduced in error
by commit 07af4ed due to a branch management error and hastily
removed again in 29e8c24. This time it really does match reality.)