work around a bug in dtrace - it strips const from function parameters

work around a bug in dtrace - it strips const from function parameters

With usedtrace this caused:

- build failures under g++ (OS X)

- const qualification warnings with gcc (OS X)

The workaround simply adds const to char * parameters if not already
present.

The bug in dtrace was apparently reported to Sun in 2006:
  http://opensolaris.org/jive/thread.jspa?threadID=53810

Mozilla ran into the same problem:
  https://bugzilla.mozilla.org/show_bug.cgi?id=593483