libmtrace: Disable writing to stdout by default.
    
    The tracing should conservatively attempt to be as transparent as
    possible and this includes avoiding writing to stderr unless in the
    event of a catastrophic error during tracing.  The early messages
    which were printed when tracing started, and those when tracing
    stops are no longer printed by default.  To print any informative
    messages the user must set the environment variable MTRACE_CTL_VERBOSE
    to 1 as documented in README.md.