Add FreeBSD support (implement SCM_CREDS/cmsgcred).

[grembo]

Depends on https://github.com/jnr/jnr-posix/pull/150 to work at runtime.

• This implements sending/receiving credentials using in a cmsgcred struct like dbus does on FreeBSD.
• Also stops using abstract in DirectConnection on FreeBSD (macOS might have the same issue).
• Tried to centralize and contain anything specific to the platform in FreeBSDHelpers.java.
• All unit tests pass on FreeBSD, assuming that a patched version of jnr-posix is used.
• Tested using signal-cli (see https://github.com/AsamK/signal-cli/issues/287).
• Probably has style issues.