libssh: new package

libssh2 and libssh both implement SSH and provide a library API for apps.
Both support SSH, SFTP, auth, channels etc. Both are 25K-30K lines of code.

[libssh2 vs libssh - A comparison]
http://www.libssh2.org/libssh2-vs-libssh.html



[Peter: add sha256 hash, fix typo in help text]
Signed-off-by: Scott Fan <fancp2007@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>