Update version number for 0.74 release.

Update version number for 0.73 release.

Update version number for 0.72 release.

Update version number for 0.71 release.

Update version number for 0.70 release.

Update version number for 0.69 release.

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 07af4ed due to a branch management error and hastily
removed again in 29e8c24. This time it really does match reality.)

Update version number for 0.67 release.

Update version number for 0.66 release.

Bump version number for 0.65 release.