Fix CGI form/cookie data handling and CGI URL encode/decode

- CGIFormData: fix broken setSize()+read() double-length bug that
  corrupted every parsed body with a leading run of garbage bytes
  (setSize(contentLength) pre-set length before read()/write()
  appended on top of it); switch content-type checks from exact
  equality to startsWith() since a real multipart Content-Type
  always carries a boundary= parameter; implement the previously
  empty multipart/form-data branch (ordinary fields only, file
  uploads deferred to a separate upload mechanism per TODO.txt);
  restore operator[] real lookup logic (was hardcoded to echo the
  key back); default-initialize formdata to NULL; fix mismatched
  include guard (leftover from an IMFFormData.h copy-paste); drop
  the unused IMFHeader.h include that pulled in a broken,
  unrelated StreamReader.h dependency.
- CGIFormattedData: parameterize the split delimiter (was hardcoded
  to &, which silently broke cookie parsing since HTTP_COOKIE uses
  ; not &) and trim whitespace on each split token.
- MString::toCGI()/fromCGI(): were unimplemented stubs (always
  returned an empty string); implemented proper CGI percent-encode
  / percent-decode + space plus-encoding, and fixed the one caller
  that discarded fromCGIs return value instead of using it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CGIFormData.h

@@ -1,5 +1,5 @@
 #ifndef __CGIFormData_h__
-# define __IMFFormData_h__
+# define __CGIFormData_h__
 
 #include "MString.h"
 #include "ZString.h"
@@ -15,7 +15,7 @@ namespace coreutils {
       MString operator[](ZString name);
       
     private:
-      CGIFormattedData *formdata; 
+      CGIFormattedData *formdata = NULL;
       MString contentType;
       
    };