Assert SSH_AUTH_SOCK exist after parse_args()

scap --help will bails out with an assertion error if one does not have
the env variable SSH_AUTH_SOCK set.  Moved the assert after parse_args()
execution so it can show the help message.

Mention in the help message that a ssh-agent is required.


Change-Id: I519425f496f78dd28e7634736daa5a0e544559d5