FreeBSD module --enable-debug --enable-invariants

Wire up the --enable-debug flag for configure to the FreeBSD module
build. Add --enable-invariants.

The running FreeBSD kernel config is used to detect whether to enable
INVARIANTS if not explicitly specified with --enable-invariants or
--disable-invariants.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11678