ip: link: Unify link type help functions a bit

ip: link: Unify link type help functions a bit

Take help function in iplink_bridge.c as an example and make other link
types' help functions similar:

* Use a single fprintf() call (if possible).
* Don't state a full command line, just "... type OPTIONS".
* Put every option in it's own line, align options by column.
* List mandatory options first.

link_veth.c is intentionally left untouched because it's 'peer' option
eats all kinds of generic link options and the help text points this out
without duplicating all the options there again.

Signed-off-by: Phil Sutter <phil@nwl.cc>