Add shlex.quote function, to escape filenames and command lines (#9723).

This function used to live as pipes.quote, where it was undocumented but
used anyway.  (An alias still exists for backward compatibility.)  The
tests have been moved as is, but the code of the function was changed to
use a regex instead of a loop with string comparisons (at Ian Bicking’s
suggestion).  I’m terrible at regexes, so any feedback is welcome.