dwatch(1): Add `-dev' option to aid debugging of profiles

The options -d' (debug), -e' (exit after compile), and `-v' (verbose)
when combined in any order (though best remembered as `-dev') will run
the conflated script through dtrace(1), test for error conditions, and
show the line that dtrace(1) failed at (with context).

If no errors are found, the output is the same as `-e[v]'.

When writing a new profile for dwatch(1), you can quickly test to
make sure it compiles by running `dwatch -devX profile_name' where
profiles live in /usr/libexec/dwatch or /usr/local/libexec/dwatch
(the latter being where profiles installed via ports should go).

Sponsored by: Smule, Inc.