Add "headerscheck" script to test header-file compilability under C.

We already had "cpluspluscheck", which served the dual purposes of
verifying that headers compile standalone and that they compile as C++.
However, C++ compilers don't have the exact same set of error conditions
as C compilers, so this doesn't really prove that a header will compile
standalone as C.

Hence, add a second script that's largely similar but runs the C
compiler not C++.

Also add a bit more documentation than the none-at-all we had before.

Discussion: https://postgr.es/m/14803.1566175851@sss.pgh.pa.us
