system: fix treatment of RT_TABLE_MAIN

system: fix treatment of RT_TABLE_MAIN

Do not treat RT_TABLE_MAIN as RT_TABLE_UNSPEC in system_resolve_rt_table() in
order to allow ip rules with lookup main to work as expected.

Provide a new function system_is_default_rt_table() to allow calling code to
specifically test for RT_TABLE_MAIN, this is going to be needed for the
backwards compatible handling of the table attribute in route objects.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>