gpg_requirement: add standalone requirement

GPG 1.x has stopped receiving new features, some of which we may well want to
take advantage of sooner or later in Homebrew. Upstream has also been attempting
to work out for a while how well used it still is which suggests it may "go away"
at some point in the future.

Debian is also in the process of migrating GnuPG 1.x to a `gpg1` executable
whilst GnuPG 2.1.x assumes the `gpg` executable. There's a detailed video
discussion of this from DebConf 2015 at:
http://meetings-archive.debian.net/pub/debian-meetings/2015/debconf15/GnuPG_in_Debian_report.webm

It's unsafe to assume every `gpg` executable is going to forever equal 1.x and
every `gpg2` executable is going to forever equal 2.x. MacGPG2 has been symlinking
2.x as a vanilla `gpg` for a while, for example, and we will be soon as well.

You'll still be able to plonk the `libexec/bin` path of `gpg` in your PATH to
access a vanilla `gpg` 1.x executable if you want to, but we're not going to
actively keep adding gpg1 support to formulae going forwards. There's really no
reason why 99.9% of projects should not or cannot use `gpg2` these days.

This uses detection methods to determine regardless of what the executable
is called we're always hitting a 2.0 GnuPG or nothing.

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