netifd: fix handling of "lookup main" for ip rules (#15315)

Reworks the handling of RT_TABLE_MAIN in system-linux.c so that ip rules
with lookup main can be properly setup.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41282 3c298f89-4303-0410-b956-a3cf2f4a3e73