gpg: set pinentry-mode to loopback

this is the missing piece for getting pinentry in minibuffer in emacs,
for example in eshell, instead of a 'pop up' pinentry that completely
messes everything up.

with this in place, don't greatly extend the key cache ttls, since we
don't have to worry about our tty emacs being interrupted if we
mindlessly stumble into a gpg pinentry.