Add more options to gpg-agent-spawn.sh, and remove GPG_TTY environment var.

Add more options to gpg-agent-spawn.sh, and remove GPG_TTY environment var.

The point of GPG_TTY is to tell gpg-agent what terminal to steal with
the pinentry program.  Because I like to keep my pinentry bound to the
terminal from which I ran gpg-agent-spawn.sh, it's easier to lock
gpg-agent into that terminal when I spawn it, and avoid reliance on
other terminals setting GPG_TTY appropriately.  This should also avoid
problems with GPGME, which seems to ignore GPG_TTY.