ctype: wrap isprint and isgraph properly.

Close #160.
* specs/posix_spec.yaml (isprint, isgraph): Move from here...
* specs/posix_compat.yaml (isprint, isgraph): ...to here.  Add
result specifications.
* specs/posix_ctype_spec.yaml (isprint, isgraph): Specify correct
thin binding for these functions.
* specs/specs.mk (specl_SPECS): Add posix_ctype_spec.yaml.
* ext/posix/ctype.c (isprint, isgraph): Return the actual result
of calling these functions.
* lib/posix/compat.lua (isprint, isgraph): Rewrite the weird true/
false return API here for backwards compatibility.

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