Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

tests/sys/net/epair: don't leak the interface after the test

Skip if_epair regression test if module doesn't exist

Approved by: kp
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D23876

tests: epair: Don't fail if we load if_epair

kldload() returns a positive integer when it loads a ko, so check that the
return value is -1 to detect error cases, not that it's different from zero.

MFC after: 3 days
X-MFC-With: r357234

tests: epair: Don't fail if the if_epair module is already loaded

kldload() returns an error (EEXIST) if the module is already loaded.
That's not a problem for us, so ignore that error.

While here also clean up include statements.

MFC after: 3 days
X-MFC-With: r357234

tests: Test for an epair panic

if_epair abused the ifr_data field to insert its second interface in
IFC_IFLIST. If userspace provides a value for ifr_data it would get
dereferenced by the kernel leading to a panic.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 3 days