Test various header files to ensure they can be included by themselves.

A number of header files in sys/* have, going back to 7th Edition Unix
in 1979, reqiured other files (like sys/types.h) to compile. Likewise
the 4BSD networking code has had prerequisites. However, going back to
around the turn of the 21st century, other systems have made them be
independently include-able (wide-spread header include protection
post-dates 7th edition Unix by maybe 3 or so years judging from USENET
source postings). Start down the path of making them all independently
include-able by creating this test that fails buildworld when they are
not.

The file 'badfiles.inc' contains a list of the currently broken files
that cannot be included w/o any prerequisites. As files are fixed, 'make
badfiles.inc' should be re-run to remove them from the list. Note: All
files that start with an underscore are considered internal and not
tested.

Please note: once a file is removed from badfiles.inc, it must pass on
all architectures. Buildworld through at least the _includes target is
needed to ensure its working (though a buildkernel should also be done
on all architectures as well).

Sponsored by: Netflix
Reviewed by: brooks, markj
Differential Revision: https://reviews.freebsd.org/D32498