link dump filter

This patch avoids a full link wildump request when the user has specified
a single link. Uses RTM_GETLINK without the NLM_F_DUMP flag.

This helps on a system with large number of interfaces.

This patch currently only uses the link ifindex in the filter.
Hoping to provide a subsequent kernel patch to do link dump filtering on
other attributes in the kernel.

In iplink_get, to be safe, this patch currently sets the answer buffer
size to the max size that libnetlink rtnl_talk can copy. The current api
does not seem to provide a way to indicate the answer buf size.

changelog from RFC to v1:
    - incorporated comments from stephen (fixed comment and fixed if/else block)

changelog from v1 to v2:
    - fix whitespaces error

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>