signing-party: optionally avoid conflicting with Mutt

Mutt installs its own private copy of pgpring.  Provide an option to
rename signing-party's to pgppubring to avoid conflicting with Mutt's.
This is the same way that FreeBSD's signing-party port works around
the conflict.
mutt: don't conflict with non-conflicting signing-party

Mutt can coexist with signing-party if signing-party is brewed
with the --with-rename-pgpring option.