MFC r345644,r346061:

MFC r345644,r346061:

r345644 (by olivier):

Skip this test if if_tap module is not available

PR:		236842

r346061:

Polish netmap(4) testcases a bit

1. Not all kernels have netmap(4) support. Check for netmap(4) support before
   attempting to run the tests via the `PLAIN_REQUIRE_KERNEL_MODULE(..)` macro.
2. Libraries shouldn't be added to LDFLAGS; they should be added to LIBADD
   instead. This allows the build system to evaluate dependencies for sanity.
3. Sort some of the Makefile variables per bsd.README.

1., in particular, will resolve failures when running this testcase on kernels
lacking netmap(4) support, e.g., the i386 GENERIC kernels on ^/stable/11 and
^/stable/12.

PR:		237129