Add @PATH bro script macro.  Addresses #869.

The macro expands to a string value containing the file system path
in which the script lives.
Make @PATH always return absolute path.  Addresses #869.
Change @PATH to @DIR for clarity.  Add @FILENAME.  Addresses #869.

@DIR expands to directory path of the script, @FILENAME expands to just
the script file name without path.
Merge remote-tracking branch 'origin/topic/jsiwek/869'

* origin/topic/jsiwek/869:
  Change @PATH to @DIR for clarity.  Add @FILENAME.  Addresses #869.
  Make @PATH always return absolute path.  Addresses #869.
  Add @PATH bro script macro.  Addresses #869.

Closes #869.