Allow to configure /var/run/netns directory

Allow to configure /var/run/netns directory

Currently NETNS_RUN_DIR is hardcoded and refers to /var/run/netns.
However, some systems (e.g. Android) doesn't have /var
which results in error attempts to create network namespaces on these
systems.  This change makes NETNS_RUN_DIR configurable at build time
by allowing to pass environment variable to make command.
Also, this change makes /etc/netns directory configurable through
NETNS_ETC_DIR environment variable.

For example: ./configure && NETNS_RUN_DIR=/mnt/vendor/netns make

Tested: verified that iproute2 with configuration mentioned above
creates namespaces in /mnt/vendor/netns

Signed-off-by: Pavel Maltsev <pavelm@google.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>