pacman-key: rework importing distro/repo provided keyrings

pacman-key: rework importing distro/repo provided keyrings

The current --reload option, apart from being non-clear in its naming,
is very limited in that only one keyring can be provided.  A distribution
may want to provide multiple keyrings for various subsets of its
organisation or custom repo providers may also want to provide a keyring.

This patch adds a --populate option that reads keyrings from (by default)
/usr/share/pacman/keyrings.  A keyring is named foo.gpg, with optional
foo-revoked file providing a list of revoked key ids.  These files are
required to be signed (detached) by a key trusted by pacman-key, in
practice probably by the key that signed the package providing these
files. The --populate flag either updates the pacman keyring using all
keyrings in the directory or individual keyrings can be specified.

Signed-off-by: Allan McRae <allan@archlinux.org>