bgpd, lib: Remove SO_MARK

bgpd, lib: Remove SO_MARK

The SO_MARK socket option was being used pre vrf to allow for the
separation of the front panel -vs- the management port.  This
was facilitated by a ip rule.  Since this is undocumented anywhere
in our system( other than old commits see
ed40466af80c9d0b88436c637a1d54b28a669b1c ).  We should remove this
because this will cause interference with people using rules
and are not aware of this offshoot of functionality.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>