exterror: Add EXTERROR_VERBOSE env variable to control verbosity

If the variable is set and the process is not suid, __uexterr_format(),
used by err(3), prints errno/category/source line/pX always, not only
when there is no kernel message provided.

Requested by: mckusick
Reviewed by: emaste, mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D54380