backends: restore shlex quoting of MESONINTROSPECT

[joukewitteveen]

The type of quoting was changed in 522392e to one that is suitable for use with cmd.exe on Windows. However, the documentation states that the type of quoting in MESONINTROSPECT is compatible with shlex.split() and elsewhere in the code, the same variable is still quoted with shlex.quote(). As mostly identified in #12148, there are a few choices:

1. Use shlex.quote() consistently and support Python but not cmd.exe.
2. Use join_args and support cmd.exe but not Python.
3. Use join_args and support splitting through the mesonbuild Python library.

This commit implements the first option and reverts part of 522392e.

Regression testing is implemented in #12115.

Fixes #12148

[joukewitteveen]

The ' '.join(shlex.quote(x) for x in ...) code occurs quite often. Maybe we should make a mesonlib function for it as well? It would be a sibling of join_args and maybe prevent people like me from being tempted to 'clean up' and just use join_args in the future.

[eli-schwartz]

It's really just a reimplementation of python 3.8's shlex.join(). I think a suitable internal name would be mesonlib.join_unix_args, which we might or might not want to differentiate from join_native_args (by renaming join_args). Depends on whether we figure that potential contributors will understand by default that join_args is native by default and join_unix_args is for when you specifically want unix style, which does seem like a reasonable assumption.

[joukewitteveen]

join_unix_args is for when you specifically want unix style

We can maybe do a bit better here, because crucially this style will be easy to use from Python as well (not just from a Unix shell). That makes it sensible for use on Windows too (e.g. this whole MESONINTROSPECT business). At any rate, this can be done after merging the current pull request, which fixes a real bug.