Add parent process ID support for command ps,
get from "/proc/<PID>/group/status:Parent".
For example
nsh> sh
nsh> ps
PID PPID GROUP CPU PRI POLICY TYPE NPX STATE EVENT SIGMASK STACK COMMAND
... ...
3 0 3 --- 100 RR Task - Waiting Signal 0000000000000000 0008136 nsh_main
... ...
9 3 9 0 100 RR Task - Running 0000000000000000 0004064 sh
nsh> sleep 100 &
sh [10:100]
nsh> ps
PID PPID GROUP CPU PRI POLICY TYPE NPX STATE EVENT SIGMASK STACK COMMAND
... ...
3 0 3 --- 100 RR Task - Waiting Signal 0000000000000000 0008136 nsh_main
... ...
9 3 9 0 100 RR Task - Running 0000000000000000 0004064 sh
10 9 10 --- 100 RR Task - Waiting Signal 0000000000000000 0004040 sh -c sleep
Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>