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>

MString.cpp

@@ -432,13 +432,57 @@ namespace coreutils {
    
    
    MString MString::toCGI() {
+      push();
+      reset();
       MString target;
+      while(!eod()) {
+	 char ch = nextChar();
+	 if(ch == ' ') {
+	    target.write('+');
+	 } else if(isalnum((unsigned char)ch) || ch == '-' || ch == '_' || ch == '.' || ch == '~') {
+	    target.write(ch);
+	 } else {
+	    target.write('%');
+	    target.write(hexChar((ch >> 4) & 15));
+	    target.write(hexChar(ch & 15));
+	 }
+      }
+      pop();
       return target;
    }
-   
-   
+
+
    MString MString::fromCGI() {
+      push();
+      reset();
       MString target;
+      while(!eod()) {
+	 char ch = nextChar();
+	 if(ch == '+') {
+	    target.write(' ');
+	 } else if(ch == '%' && !eod()) {
+	    char ch1 = nextChar();
+	    ch1 -= 48;
+	    if(ch1 > 9)
+	      ch1 -= 7;
+	    ch1 <<= 4;
+	    ch1 &= 240;
+	    if(!eod()) {
+	       char ch2 = nextChar();
+	       ch2 -= 48;
+	       if(ch2 > 9)
+		 ch2 -= 7;
+	       ch2 &= 15;
+	       ch1 |= ch2;
+	       target.write(ch1);
+	    } else {
+	       target.write(ch1);
+	    }
+	 } else {
+	    target.write(ch);
+	 }
+      }
+      pop();
       return target;
    }