datatype: rt_symbol_table_init() to search for iproute2 configs

datatype: rt_symbol_table_init() to search for iproute2 configs

There is an ongoing effort among various distributions to tidy up in
/etc. The idea is to reduce contents to just what the admin manually
inserted to customize the system, anything else shall move out to /usr
(or so). The various files in /etc/iproute2 fall in that category as
they are seldomly modified.

The crux is though that iproute2 project seems not quite sure yet where
the files should go. While v6.6.0 installs them into /usr/lib/iproute2,
current mast^Wmain branch uses /usr/share/iproute2. Assume this is going
to stay as /(usr/)lib does not seem right for such files.

Note that rt_symbol_table_init() is not just used for
iproute2-maintained configs but also for connlabel.conf - so retain the
old behaviour when passed an absolute path.

Signed-off-by: Phil Sutter <phil@nwl.cc>