When passing an empty string as a directory, the function would produce
filenames starting with a slash even when the given file_name is not an absolute
path. Defaulting to the root directory is likely never intended and might
conveivably be dangerous. The middle "/" is now skipped also if dir is an empty
string.
This also installs symlinks from "zeek" and "bro-config" to a wrapper
script that prints a deprecation warning.
The btests pass, but this is still WIP. broctl renaming is still
missing.
#239
