Rearrange traceflags.py so that the file generation only happens if

Rearrange traceflags.py so that the file generation only happens if
the script is invoked as main.  This allows us to import traceflags.py
if we just want the list of available flags.
Embed traceflags.py into the zipfile so it can be accessed from the
python side of things.  With this, print an error on invalid flags and
add --trace-help option that will print out the list of trace flags
that are compiled in.  If a flag is prefixed with a '-', now that flag
will be disabled.

--HG--
extra : convert_revision : 2260a596b07d127c582ff73474dbbdb0583db524