Fix jet cgi=yes CGI variable loading and related bugs

- __jet.cpp: the cgi keyword check compared against "true" but the
  documented/intended value is "yes", so <jet cgi="yes" ...> never
  actually turned CGI mode on; fixed. Also replaced the hardcoded
  fake multipart boundary ("xxxx") with real extraction from the
  Content-Type header's boundary= parameter.
- Global.h: default-initialize cgiData to NULL (a GET request with
  cgi="yes" never allocates it, so it was previously left
  dangling); Global.cpp now sets the cookie jar's delimiter to ";"
  to match RFC 6265, since it defaulted to CGIFormattedData's
  &-based form-data delimiter.
- Global.cpp: fix addSession()'s missing throw that let it silently
  continue past a duplicate session id.
- Tag.cpp: fix the CGI-variable lookup branch, which failed to
  compile (cgiData is a pointer; needed to dereference it before
  indexing) and added a null check so a CGI request that never
  populated cgiData (bad Content-Type, or the null-safety gap
  above) throws a clear error instead of crashing; implemented the
  previously-empty session-variable lookup branch; left the
  unreachable duplicate environment-variable branch as-is with a
  comment, since fixing its sigil is a user-facing template-syntax
  decision, not a bug fix.

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

Global.cpp

@@ -7,7 +7,7 @@
 namespace jet {
 
    Global::Global(char **envp) : envp(envp) {
-      
+      cookies.setDelimiter(";");
    }
 
    Global::~Global() {
@@ -25,7 +25,7 @@ namespace jet {
    
    void Global::addSession(coreutils::MString sessionId, __mysql *mysql) {
       if(sessionExists(sessionId))
-	coreutils::Exception("sessionid already exists.");
+	throw coreutils::Exception("sessionid already exists.");
       sessions[sessionId] = mysql;
    }