fairland: Make write_newcap_script() failure fatal

fairland: Make write_newcap_script() failure fatal

When write_newcap_script() fails, it complains to stderr and fails.
main() doesn't bother to check for failure.  Has always been that way.
Fix main() to check.  Also adjust write_newcap_script() to return one
on success, zero on failure, like the other functions do.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>