acinclude.m4: improve autodetection of CA bundle on FreeBSD

The FreeBSD Port security/ca_root_nss installs the Mozilla NSS CA bundle
to /usr/local/share/certs/ca-root-nss.crt. Use this bundle in the
discovery process.

This change also removes the former FreeBSD path that has been obsolete
for 8 years since this FreeBSD ports commit:
https://svnweb.freebsd.org/ports/head/security/?view=revision&revision=215953

Closes #894