Implement BROWSE on Mac OS X.

This commit consists of two changes:

1.  The system(3) call in `Try_Browser' has been replaced with
    fork(2) and execlp(3). As such, the call string needn't be
    constructed and failed calls no longer print their output to the
    console.

2.  On OS X, `OS_Browse' only attempts to use `/usr/bin/open'; on
    other POSIX platforms, "xdg-open" and "x-www-browser" are used
    as before.