Make sure the rtld(1) error messages go to stderr, not stdout.
While here fix capitalization of a few nearby strings, add the
rtld's file name prefix so it's obvious where the message come
from, and return zero when "-h" is used.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16530