posix_spawn[p]: Don't execute scripts without '#!' marker through /bin/sh

[Bruno Haible]

The posix_spawn implementation in gnulib comes from glibc as of 2008.
But in 2011 an important change was done in glibc: Remove the ability
to specify a script that does not start with a '#!' marker as executable.
Previously this file was /_assumed_/ to be a shell script.

Here are 3 patches:
  - Change gnulib's posix_spawn to match what glibc does.
  - Add unit tests.
  - Fix the resulting test failures on many platforms (from GNU/Hurd [1] to
    Solaris 11).

[1] https://lists.gnu.org/archive/html/bug-hurd/2020-12/msg00071.html


2020-12-23  Bruno Haible  <bruno@clisp.org>

        posix_spawn, posix_spawnp: Fix execution of scripts.
        * m4/posix_spawn.m4 (gl_POSIX_SPAWN_SECURE): New macro.
        (gl_POSIX_SPAWN_BODY): Invoke it. Set REPLACE_POSIX_SPAWN if posix_spawn
        or posix_spawnp allows unsecure execution of scripts.
        * doc/posix-functions/posix_spawn.texi: Document the script execution
        problem.
        * doc/posix-functions/posix_spawnp.texi: Likewise.

2020-12-23  Bruno Haible  <bruno@clisp.org>

        Add unit tests regarding execution of scripts.
        * tests/executable-script: New file.
        * tests/executable-shell-script: New file.
        * tests/test-posix_spawn-script.c: New file.
        * tests/test-posix_spawnp-script.c: New file.
        * tests/test-execute-script.c: New file.
        * tests/test-spawn-pipe-script.c: New file.
        * modules/posix_spawn-tests (Files): Add
        tests/test-posix_spawn-script.c, tests/executable-script,
        tests/executable-shell-script.
        (Makefile.am): Compile and run test-posix_spawn-script.
        * modules/posix_spawnp-tests (Files): Add
        tests/test-posix_spawnp-script.c, tests/executable-script,
        tests/executable-shell-script.
        (Makefile.am): Compile and run test-posix_spawnp-script.
        * modules/execute-tests (Files): Add tests/test-execute-script.c,
        tests/executable-script, tests/executable-shell-script.
        (Makefile.am): Compile and run test-execute-script.
        * modules/spawn-pipe-tests (Files): Add tests/test-spawn-pipe-script.c,
        tests/executable-script, tests/executable-shell-script.
        (Makefile.am): Compile and run test-spawn-pipe-script.

2020-12-23  Bruno Haible  <bruno@clisp.org>

        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.

0001-Don-t-execute-scripts-without-marker-through-bin-sh.patch
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0002-Add-unit-tests-regarding-execution-of-scripts.patch
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0003-posix_spawn-posix_spawnp-Fix-execution-of-scripts.patch
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