Revision 1796208

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