atf, kyua: Implement require.kmods.

This adds a metadata variable, require.kmods, and corresponding functions
or methods in C, C++, and shell, which allow a test to specify that it
requires particular kernel modules to run. If the kernel modules are not
present, the test is skipped. One might want to consider a kyua option
which makes it attempt to load the modules instead.

Differential Revision: https://reviews.freebsd.org/D47470