regex related cleanups:

- use AP_MAX_REG_MATCH where appropriate
- in mod_proxy_ftp, compile the "ls -s1" regex only once instead of once per request
- add some coments


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1185898 13f79535-47bb-0310-9956-ffa450edef68