This (1) fixes an issue where signature files supplied on the command
line wouldn't pass through the hooks, and (2) prepares for allowing
hooks to supply the content of a signature file directly.
For backward compatibility when reading values, we first check
the ZEEK-prefixed value, and if not set, then check the corresponding
BRO-prefixed value.
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
