if_epair: support IFCAP_MEXTPG

Enable IFCAP_MEXTPG by default, which may bring performance benefits.
Allow it to be disabled, and when disabled assert that we do not receive
any mbufs with M_EXTPG set. This is useful for testing.
Default the tests to disabling MEXTPG support.

Reviewed by: zlei
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D58054