lib: parser: support keyword arguments

lib: parser: support keyword arguments

This re-adds "{foo WORD|bar WORD}" keyword-argument support to the CLI
parser.  Note that token graphs may now contain loops for this purpose;
therefore the matching functions retain a history of already-matched
tokens.  Each token can thus only be consumed once.

And then LINE... gets its special treatment with allowrepeat.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>