[SV 57022] Avoid posix_spawn which fails asynchronously

[SV 57022] Avoid posix_spawn which fails asynchronously

Avoid using posix_spawn implementations that fail asynchronously when
the spawned program can't be invoked: this means instead of getting
an error such as "No such file or directory" we get just "Exit 127".

Original implementation of the configure.ac macro provided by
Martin Dorey <martin.dorey@hds.com>

Original implementation of the regression tests provided by
Dmitry Goncharov <dgoncharov@users.sf.net>

* configure.ac: Test whether posix_spawn fails asynchronously.  In a
cross-compilation environment, assume that it does not.  If we detect
that it does, fall back to fork/exec.
* tests/scripts/features/exec: Add regression tests for different
shebang invocation methods.