docgen now lists macro implementation status in the generated rst files.
Adds POSIX definition link property to docgen json API (`posix-definition`) and
changes the `defined` property of docgen json API to `c-definition`. Now that
docgen's api is getting more specified, adds validation checks to docgen to
start codifying the docgen api spec.
To make sure this all looks good, I've added POSIX definition links to signal.h
as a tester.
