msgs: Use C99 bool instead of defining our own

Use stdbool.h definitions instead of defining non-standard ones.

Signed-off-by: Collin Funk <collin.funk1@gmail.com>
Pull-request: https://github.com/freebsd/freebsd-src/pull/1105
(cherry picked from commit 912b52a62c72ca9c96a396148a6b30ff9c0917e7)