treewide: refactor help messages

treewide: refactor help messages

Every tool in the iproute2 package have one or more function to show
an help message to the user. Some of these functions print the help
line by line with a series of printf call, e.g. ip/xfrm_state.c does
60 fprintf calls.
If we group all the calls to a single one and just concatenate strings,
we save a lot of libc calls and thus object size. The size difference
of the compiled binaries calculated with bloat-o-meter is:

        ip/ip:
        add/remove: 0/0 grow/shrink: 5/15 up/down: 103/-4796 (-4693)
        Total: Before=672591, After=667898, chg -0.70%
        ip/rtmon:
        add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-54 (-54)
        Total: Before=48879, After=48825, chg -0.11%
        tc/tc:
        add/remove: 0/2 grow/shrink: 31/10 up/down: 882/-6133 (-5251)
        Total: Before=351912, After=346661, chg -1.49%
        bridge/bridge:
        add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-459 (-459)
        Total: Before=70502, After=70043, chg -0.65%
        misc/lnstat:
        add/remove: 0/1 grow/shrink: 1/0 up/down: 48/-486 (-438)
        Total: Before=9960, After=9522, chg -4.40%
        tipc/tipc:
        add/remove: 0/0 grow/shrink: 1/1 up/down: 18/-62 (-44)
        Total: Before=79182, After=79138, chg -0.06%

While at it, indent some strings which were starting at column 0,
and use tabs where possible, to have a consistent style across helps.

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David Ahern <dsahern@gmail.com>