Test for Apache executable in sbin dirs

Debian apparently doesn't include /usr/sbin in the default user PATH, so
add an APXS2-like lookup with likely locations if we don't find the
apache executable in our standard path.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Jason Etheridge <jason@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>