Only read states of specified process when refresh_processes_specifics

[xiaopeng8570]

When I call refresh_processes_specifics with specified pid, the function refresh_procs in process.rs always get all pid entries, then filter the specified process out.
Maybe it is not efficient for the case that I only need one process's information.
It will be much more slower with reading every task files of all processes, as issue #1432 says.

[sigsegved]

@xiaopeng8570 does this mean it reads stat files for all pids and then filters the result? that does seem super wasteful. I didnt realize that was the case.