fixproto: Define NULL and size_t in generated stdlib.h and unistd.h.

fixproto: Define NULL and size_t in generated stdlib.h and unistd.h.

	* fixproto: Define NULL and size_t in generated stdlib.h and
	unistd.h.  Kill unused required_stdlib_h, required_unistd_h.
	Rearrange file generation loop for readability.  Generate time.h,
	string.h if missing.
	* tsystem.h: Include <string.h>, <time.h> unconditionally.
	* config.gcc: Blow away POSIX defines.

From-SVN: r64690