lib, ospfd, ripd, ripngd: Fix 'set metric'

lib, ospfd, ripd, ripngd:  Fix 'set metric'

There are a variety of cli's associated with the
'set metric ...' command.  The problem that we
are experiencing is that not all the daemons
support all the varieties of the set metric
and the returned of NULL during the XXX_compile
phase for these unsupported commands is causing
issues.  Modify the code base to only return
NULL if we encounter a true parsing issue.
Else we need to keep track if this metric
applies to us or not.

In the case of rip or ripngd if the metric
passed to us is greater than 16 just turn
it internally into a MAX_METRIC.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>