Changes to improve compatibility with OS X

On Macs, both with system tools and brew, glibtoolize exists but libtoolize does not. I've adjusted the autogen.sh script to account for this so it works in either case.

In the config file (.fwknoprc), there didn't appear to be a way to enable gpg-agent usage. I've added a "USE_GPG_AGENT" option to allow this to be set via the config file.

OSX appears to fail when trying to stat a directory which needs to be expanded, such as "~/.gnupg". This causes the validation check to fail. I've adjusted the check to first expand the path to resolve this issue.

I'm available on Freenode in #cipherdyne if there's anything I need to adjust/fix to get this pulled in.