mod_setenvif: Compile the regex used by is_header_regex() only once

during startup This should save some memory, especially with .htaccess

Backport of r1343099.

Submitted by: sf
Reviewed by: rjung, jim
Backported by: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1365774 13f79535-47bb-0310-9956-ffa450edef68