util.c: add a strict Base64 decoding function

ap_pbase64decode_strict() adds to the functionality of
ap_pbase64decode() in two ways:

- the length of the decoded buffer is returned, allowing embedded NULLs
  to be retained by the caller
- the input string is strictly checked for Base64 validity, including
  correct zero-padding at the end of the string

(This was originally added to the httpdunit feature/backport branch in
r1796208, then reverted in r1799376, since it's currently intended for
trunk only.)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1799380 13f79535-47bb-0310-9956-ffa450edef68