xargs: fix -R so that it accepts negative numbers again

fbc445addf9 converted the parsing of arguments to strtonum but made
the accepted range for -R too restrictive. As documented in the man
page, it should accept negative numbers.

Added a test for this, which was provided by Jose Luis Duran.

Fixes: fbc445addf9
MFC after: 1 week
Reviewed by: des, kevans
Differential Revision: https://reviews.freebsd.org/D41021