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