odhcpd: change "-c" cmd line arg to take a dir

odhcpd: change "-c" cmd line arg to take a dir

After the TZ support was added, odhcpd now reads two cfg files
potentially. Instead of adding a separate switch for the system UCI
file, I've changed the "-c" argument to take a directory. This is more
future-proof and also serves as a preparation for the global DUID PR
(which will necessitate reading from the UCI "network" file as well).

While I was at it, I also made some tiny improvements to the
odhcpd_reload() function to avoid an unnecessary allocation.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/291
Signed-off-by: Álvaro Fernández Rojas <[email protected]>