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

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/httpdunit@1796208 13f79535-47bb-0310-9956-ffa450edef68