scan_base64url - decode base64url encoded data

       base64url  is a variant of base64 for use in URLs (standard base64 uses / and +, which can cause problems
       in URLs, so base64url uses - and _ instead; also base64url does not use = padding at the end).

       scan_base64url decodes base64url encoded data from src into dest.  It will stop when  it  encounters  any
       non-valid  input  characters.   It will then write the number of decoded bytes in dest into *destlen, and
       return the number of bytes decoded from src.

       Many base64 variants demand padding in the last block.  Some  don't.  This  implementation  will  consume
       padding if it is there, but will not complain if it is not.

       dest can be NULL. destlen can be NULL. dest can be src.

       scan_base64url("Zm5vcmQ",buf,&i) -> return 7, i=5, buf="fnord"

       scan_base64url - decode base64url encoded data

       scan_base64url returns the number of bytes successfully scanned and processed from src.

scan_base64(3), scan_xlong(3), scan_8long(3), fmt_ulong(3)

                                                                                               scan_base64url(3)

#include<libowfat/textcode.h>

       size_t scan_base64url(const char *src,char *dest,size_t* destlen);