net/samba416: Add externalldb flavor

Why do we need the externalldb flavor? Compiling tools like
security/sssd-devel with Kerberos support often requires a standalone
ldb as a dependency. By adding a externalldb flavor, ports can depend on
a flavor of Samba that does not depend on its bundled version of ldb.

PR: 276835
Approved by: portmgr (maintainer timeout, 2+ weeks)
Sponsored by: Klara, Inc.
Co-authored-by: Mateusz Piotrowski <0mp@FreeBSD.org>