Make mod_substitute more efficient:

- Use varbuf resizable buffer instead of constantly allocating pool
  memory and copying data around. This changes the memory requirement from
  quadratic in ((number of substitutions in line) * (length of line)) to
  linear in (length of line).
- Instead of copying buckets just to append a \0, use new ap_regexec_len()
  function

PR: 50559


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1176019 13f79535-47bb-0310-9956-ffa450edef68