cdefs.h: Retire const and signed

These are no longer used in the base system. Once upon a time they were
used to ease the transition from K&R to ANSI C. These days, we use both
of these keywords in their bare form everywhere, even in our
headers. Any uses of const or signed will be converted to their ANSI
meaning in the compiler, but is beyond the scope of cdefs.h's
charter. For pure K&R compilation, with an enforced only use K&R
features and meanings, one needs to define NO_ANSI_KEYWORDS (though even
that support is wobbly at best, being active only for !GCC or
gcc-adjacent compilers which currently appears to be none).

Sponsored by: Netflix
Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D45860