Implement BROWSE on Mac OS X.

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.
Clean up browser calls for POSIX platforms.

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.