aux/getflags: improve flagfmt parser

aux/getflags: improve flagfmt parser

This makes the flagfmt parser more robust and accepting
a looser input language — namely by allowing whitespace
around specifier fields and ignoring any empty fields.

Long flagfmts can thus be pleasingly displayed:

	flagfmt='
		a, b, c, C:cache,
		m:mtpt mountpoint,
		s:srvn srvname'