Polish netmap(4) testcases a bit

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
Reviewed by:	vmaffione
Approved by:	emaste (mentor)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D19864