Use "mkdir -p" if it works, as that fixes bugs when making file names

Use "mkdir -p" if it works, as that fixes bugs when making file names
with unusual characters.  Quote chmod arg "$dirmode", in case it
contains white space.

Incorporate the following change from autoconf mkinstalldirs:

1999-04-10  Ben Elliston  <bje@cygnus.com>

* mkinstalldirs: Add `-m' flag to specify the mode of a newly created
directory.  Add command line usage and `-h', `--help' options.
Contributed by Jeff Garzik.