lib: make resolve_hosts variable common

lib: make resolve_hosts variable common

Any iproute utility that uses any function from lib/utils.c needs
to declare its own resolve_hosts variable instance although it does
not need/use hostname resolving functionality (currently only 'ip'
and 'ss' commands uses this).
The patch declares single common instance of resolve_hosts directly
in utils.c so the existing ones can be removed (the same approach
that is used for timestamp_short).

Cc: Jiri Pirko <jiri@mellanox.com>
Cc: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>