optimization:

- add comment about what subst_prefix_path function does
- reduce the use of fixed buffers
- get a rid of unnecessary memory operations

Justin adds:
Fixes // problem and rewrites subst_prefix_path to make it intelligble.

Backport of modules/mappers/mod_rewrite.c r1.162 from httpd-2.1
Submitted by:	Andr� Malo
Reviewed by:	Andr� Malo, Jeff Trawick, Justin Erenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@102118 13f79535-47bb-0310-9956-ffa450edef68