support to read from /proc/net/tcp{,6} (prometheus#332)

* Add GIDs to ProcStatus struct

Signed-off-by: Andrei Volnin <[email protected]>

* support to read from /proc/net/tcp{,6}

Signed-off-by: Hubert Chen <[email protected]>
Co-authored-by: Andrei Volnin <[email protected]>

Fix bcache stats_total per-device readings (prometheus#323)

Signed-off-by: Vitaliy Filippov <[email protected]>

Co-authored-by: Vitaliy Filippov <[email protected]>