FIX: add missing xml mime type
FIX: remove unwanted printf when exec fails
CLEAN
FIX: mprGetPid for VxWorks
FIX: mprGetPid for VxWorks
FIX: SIGCHLD signal handling for MprCmd

Fixed signal handling with multiple threads and programs that cannot exec()
- Add locking to mprServiceSignals
- Immediately create event for all registered signal handlers instead of chaining
- Add GC handling for safety to unlink signal handler