ps: allow large list of groups

ps: allow large list of groups

Current linux kernels output no more than 32 groups
in /proc/{pid}/status.

Plan is to increase this limit.

This patch allows ps to not core dump if the buffer used to read status
file was too small.

# ps aux
Signal 11 (SEGV) caught by ps (procps-ng version 3.3.3).
ps:display.c:59: please report this bug

Also increases the size of the buffer from 1024 to 4096, since even with
32 groups we are close to the limit.

cat /proc/12731/status | wc
     39     128     961

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>