xargs: disallow -R0 and -L0

Both cases were interpreted as these flags are unset. This meant that
-R0 got converted to -R5 and that -L0 didn't have any effect at all.
Since make at most 0 replacements isn't useful and since call utility
for every 0 lines read doesn't make sense, throw an error for these
two cases.

MFC after: 1 week
Reviewed by: des, kevans
Differential Revision: https://reviews.freebsd.org/D41022