Broxygen init fixes, addresses BIT-1110.

- Don't check mtime of bro binary if BRO_DISABLE_BROXYGEN env var set.

- Fix failure to locate bro binary if invoking from a relative
  path and '.' isn't in PATH.
Improve GeoIP City database support.

When trying to open a city database, it now considers both the "REV0"
and "REV1" versions of the city database instead of just the former.
The extra fields of the "REV1" version (metro/area code) aren't yet put
in geo_location records, this change just allows this version of the
city database to be opened w/ same functionality as the other version.
This should be convenient because libGeoIP expects either version to
live at the same file system path, it's hard to tell which version
you've got, and current free GeoLite databases seem to be "REV1".