lib/repo: Search a list of paths in gpgkeypath for gpg keys

lib/repo: Search a list of paths in gpgkeypath for gpg keys

This allows specifying gpgpath as list of
paths that can point to a file or a directory. If a directory path
is given, paths to all regular files in the directory are added
to the remote as gpg ascii keys. If the path is not a directory,
the file is directly added (whether regular file, empty - errors
will be reported later when verifying gpg keys e.g. when pulling).

Adding the gpgkeypath property looks like:

ostree --repo=repo remote add --set=gpgpath="/path/key1.asc,/path/keys.d" R1 https://example.com/some/remote/ostree/repo

Closes #773

Closes: #1773
Approved by: cgwalters