Don't execute scripts without '#!' marker through /bin/sh.
This reflects the change done in glibc through
<https://sourceware.org/bugzilla/show_bug.cgi?id=13134> and
<https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=
d96de9634a334af16c0ac711074c15ac1762b23c>.
* lib/spawni.c (internal_function): Remove macro.
(script_execute): Remove function.
(__spawni): Don't invoke script_execute.
* lib/execute.c (execute): Disable the ENOEXEC handling.
* lib/spawn-pipe.c (create_pipe): Likewise.
* NEWS: Mention the change.