[process][unix] err != nil when calling process.Children() for a process without child processes

[lichunqiang]

Describe the bug
Calling Children() on a process without child processes returns a non-nil err

the source code uses common.CallPgrepWithContext which calls pgrep -P $PID. If input pid is without child processes, pgrep exit code is 1, the function call returns error with exits status 1

To Reproduce

// paste example code reproducing the bug you are reporting

Expected behavior
The function should return ErrorNoChildren?

Environment (please complete the following information):

• Windows: [paste the result of ver]
• Linux: [paste contents of /etc/os-release and the result of uname -a]
• Mac OS: [paste the result of sw_vers and uname -a
• FreeBSD: [paste the result of freebsd-version -k -r -u and uname -a]
• OpenBSD: [paste the result of uname -a]

Additional context
[Cross-compiling? Paste the command you are using to cross-compile and the result of the corresponding go env]

[Lomanic]

To me this was introduced in #1231 (meant to handle places where pgrep is missing).

As pgrep returns 1 (without any message in stderr/out) both when there's no child process found or the parent process doesn't exist (pgrep -P $$; echo $? vs pgrep -P 3; echo $?), gopsutil should get rid of pgrep and follow psutil, like this on linux (calls ppid() on all processes on all other posix OSes).

As an aside, I think this ErrorNoChildren error should not exist in the first place, the Children() function should simply return a nil error and an empty slice as this is not an error.