execute, spawn-pipe: Use posix_spawn by default on native Windows

[Bruno Haible]

Now that posix_spawn works on native Windows, the modules 'execute'
and 'spawn-pipe' can be changed to use this as the new implementation
(instead of 'spawnvech', which implements only a special case).

This new implementation surely allocates more temporary memory, and
also calls DuplicateHandle more often than the previous implementation.
But it is more open to customizations (e.g. call setpgrp or setsid
in the future).


2020-12-24  Bruno Haible  <bruno@clisp.org>

        spawn-pipe: Use posix_spawn by default on native Windows.
        * lib/spawn-pipe.c (SPAWN_PIPE_IMPL_AVOID_POSIX_SPAWN): New macro.
        (create_pipe): Use it to decide among the two possible implementations.
        * modules/spawn-pipe (Depends-on): Add posix_spawnattr_setpgroup.

2020-12-24  Bruno Haible  <bruno@clisp.org>

        unistd-safer: Implement pipe_safer on native Windows.
        * lib/pipe-safer.c (pipe_safer): Don't test HAVE_PIPE.
        * modules/unistd-safer (Depends-on): Add pipe-posix.

2020-12-24  Bruno Haible  <bruno@clisp.org>

        execute: Use posix_spawn by default on native Windows.
        * lib/execute.c (EXECUTE_IMPL_AVOID_POSIX_SPAWN): New macro.
        (execute): Use it to decide among the two possible implementations.

0001-execute-Use-posix_spawn-by-default-on-native-Windows.patch
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0002-unistd-safer-Implement-pipe_safer-on-native-Windows.patch
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0003-spawn-pipe-Use-posix_spawn-by-default-on-native-Wind.patch
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