unistd: deprecate exec with string tuple args.

Close #167.
* specs/posix_spec.yaml (exec, execp): Move from here...
* specs/posix_deprecated.yaml (exec, execp): ...to here.
* specs/posix_unistd.yaml (exec, execp): ...and much simplified
specification here.
* ext/posix/unistd.c (runexec): Simplify accordingly.
* lib/posix/deprecated (exec, execp): Reimplement string tuple
passing, [0]-crippled API again over C binding.
* NEWS: Update.

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