Improve trace logging -

Improve trace logging -
- Add logfile and syslog capability. For Windows, the latter is simulated by writing to a file. Adding Eventlog support is for a future effort. The choice of logging mode is made via MONO_LOG_DEST environment variable.
- Add timestamps and pid to the trace messages to make them more useful
- Add timestamps to SGEN_LOG
- Related to tracing: when tracing and using something like Valgrind the system may be slowed so much that the sleep abort limit may be exceeded for threads. This fix adds an environment variable lookup of "MONO_SLEEP_ABORT_LIMIT" which is a value greater than 40, which defines the number of milliseconds to wait for threads to complete pending operations. If not set, the valus is 200.