gpg integration is improved

[litch]

We operate PRM via another controlling process (a web app) that does not have access to tty. However, without access to tty, gpg fails, and the release is not signed - leading to all kinds of problems. This was also experienced here, it seems:

#69

While adding the tty flag, we found that though gpg_passphrase is set to false by Clamp, the logic seems to be predicated on it being nil.

Result of a prm invocation without a passphrase:

sign_cmd = echo 'false' | gpg --digest-algo \"SHA256\" -u <KeyID> --passphrase-fd 0 --yes --output Release.gpg -b Release

An unprotected key doesn't seem to mind if you pass it the wrong password, so it's possible to eliminate the third branch entirely, but that seems sloppy.