resources: don't truncate rlim structure field values.

With f8d8939 we were too zealous with changing all internal
types from lua_Number (et. al) to lua_Integer (et. al), where
lua_Integer is not wide enough to contain an rlim_t.
* ext/include/_helpers (checknumberfield, pushnumberfield)
(setnumberfield): New function and macros equivalent to the
earlier implementations of checkintegerfield, pushintegerfield
and setintegerfield respectively.
* ext/posix/sys/resource.c (pushrlimit): Set PosixRlimit table
entries with setnumberfield.
(setrlimit): Extract PosixRlimit fields with checknumberfield.
* specs/posix_sys_resource_spec.yaml (setrlimit): Remove
spurious braces.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>