df(1): Allow duplicate -l flags gracefully

Rather than producing a misleading error message when duplicate -l flags are
provided to df(1), simply ignore extra flags and proceed as if only one was
specified. This seems most reasonable given the usage for -l:

-l      Only display information about locally-mounted file systems.

l and t flags still conflict, as before.

PR: 208169
Reported by: by at reorigin.com
Reviewed by: allanjude