ss: add support for cgroup v2 information and filtering

ss: add support for cgroup v2 information and filtering

This patch introduces two new features: obtaining cgroup information and
filtering sockets by cgroups. These features work based on cgroup v2 ID
field in the socket (kernel should be compiled with CONFIG_SOCK_CGROUP_DATA).

Cgroup information can be obtained by specifying --cgroup flag and now contains
only pathname. For faster pathname lookups cgroup cache is implemented. This
cache is filled on ss startup and missed entries are resolved and saved
on the fly.

Cgroup filter extends EXPRESSION and allows to specify cgroup pathname
(relative or absolute) to obtain sockets attached only to this cgroup.
Filter syntax: ss [ cgroup PATHNAME ]
Examples:
    ss -a cgroup /sys/fs/cgroup/unified (or ss -a cgroup .)
    ss -a cgroup /sys/fs/cgroup/unified/cgroup1 (or ss -a cgroup cgroup1)

v2:
  - style fixes (David Ahern)

Signed-off-by: Dmitry Yakunin <zeil@yandex-team.ru>
Signed-off-by: David Ahern <dsahern@gmail.com>