resource: bind getrlimit and setrlimit minimally.

Close #164.
* specs/posix_spec.yaml (getrlimit, setrlimit): Move from here...
* specs/posix_compat_spec.yaml (getrlimit, setrlimit): ...to here.
* specs/posix_sys_resource_spec.yaml (getrlimit, setrlimit): New
file. Specify minimal binding behaviours.
* specs/specs.mk (specl_SPECS): Add
specs/posix_sys_resource_spec.yaml.
* ext/include/_helpers.c (checkfieldtype): Improve nil diagnostic
to read 'no value'.
(lookup_symbol): Remove unused function.
* ext/posix/sys/resource.c (rlimit_map, Krlimit, Srlimit): Remove
unused internal APIs.
(pushrlimit): Push a PosixRlimit table onto the stack.
(getrlimit, setrlimit): Simplify accordingly.
* lib/posix/compat.lua (optint): New helper function.
(getrlimit, setrlimit): Reimplement string resource multi-return
API in Lua.
* NEWS: Update.

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