swconfig: add SWITCH_TYPE_LINK and support sending link info to user space

So far we were sending link data as a string. It got some drawbacks:
1) Didn't allow writing clean user space apps reading link state. It was
   needed to do some screen scraping.
2) Forced whole PORT_LINK communication to be string based. Adding
   support for *setting* port link required passing string and parting
   it in the kernel space.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
SVN-Revision: 47997