SSH2: make $mac_algorithms non-static

none of the other statically defined variables have the potential
to change as multiple Net_SSH2 objects are created but this one does.

ie. if you connect to one SSH-2.0-SSHD server then no subsequent server
will use hmac-sha1-96 even if it could
Merge pull request #367 from terrafrost/ssh-mac-algo

SSH2: make $mac_algorithms non-static

* terrafrost/ssh-mac-algo:
  SSH2: make $mac_algorithms non-static