For -l, upset everyone by breaking it. Specifically, -l now produces
a lengthy error message that suggests --check-links (POSIX -l) or
--one-file-system (GNU -l) instead. However, if POSIXLY_CORRECT is set,
use the POSIX interpretation.
For -o, please everyone by making it work both ways:
* -xo uses POSIX behavior
* -co uses "almost GNU" behavior (as close as we can get until
libarchive implements a true V7 tar format)