netifd: Add sendredirects config support

netifd: Add sendredirects config support

Setting /proc/sys/net/ipv4/conf/*/send_redirects is useful if a single
layer-2 domain is shared among routed subnets.
Sending redirects will prevents traffic from taking unnessesary detours
through a gateway in cases where direct connectivity on layer 2 exists.
This is commonly the case if an existing LAN infratructure with dump
switches is used to additionally carry routing protocols like OLSR
which are supported only by some nodes on the network.
It's important to note that the default value for send_redirects
differs for interface types (it's enabled on physical ethernet
interfaces, but disabled e.g. on VLANs) due to olsrd changing
/proc/sys/net/ipv4/conf/default/send_redirects during boot, thus the
default differs also depending e.g. on the way an on-board switch is
integrated on specific boards (as eth0 exists before olsrd is started,
eth0.1 gets created by netifd later on...)
Having a way to explicitely enable or disable send_redirects is
thus desireable also to unify the default behaviour among different,
but seemingly similar devices supported.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>