netlink-socket: Add conceptual documentation.

Based on a conversation with the VMware Hyper-V team earlier today.

This commit also changes a couple of functions that were only used with
netlink-socket.c into static functions.  I couldn't think of a reason for
code outside that file to use them.

Signed-off-by: Ben Pfaff <blp@nicira.com>
cmap: Merge CMAP_FOR_EACH_SAFE into CMAP_FOR_EACH.

There isn't any significant downside to making cmap iteration "safe" all
the time, so this drops the _SAFE variant.

Similar changes to CMAP_CURSOR_FOR_EACH and CMAP_CURSOR_FOR_EACH_CONTINUE.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>