Added pagent ssh auth support (cmderdev#1391)

(and some comments on the user-profile.cmd file)

Inspired by the comments from cmderdev#193 and my personal need to use pageant instead of OpenSSH authentication agents (which is more Window user-friendly), I have used this approach which works as expected.

Keeping the spirit of the current scripts, I left it disabled, and with some comments explaining what they all do.