check-procs: check PPID

[hfm]

I add new option for checking PPID.
It checks PPID of target processes like the following:

$ ./check-procs --ppid 1 -p bash -W 0 -C 0
Procs OK: Found 0 matching processes; cmd /bash/; ppid 1

[hfm]

check_procs_unix.go is not using func parsePerfProc() ?

https://travis-ci.org/mackerelio/go-check-plugins/builds/105369944#L331-L338

# github.com/mackerelio/go-check-plugins/check-procs
check-procs/check_procs_unix.go:67: cannot use vsz (type int64) as type string in field value
check-procs/check_procs_unix.go:67: cannot use pcpu (type float64) as type int64 in field value
check-procs/check_procs_unix.go:67: cannot use thcount (type int64) as type float64 in field value
check-procs/check_procs_unix.go:67: cannot use "" (type string) as type int64 in field value
check-procs/check_procs_unix.go:67: cannot use esec (type int64) as type string in field value
check-procs/check_procs_unix.go:67: too few values in struct initializer

[hfm]

Okay, Travis CI build passed 😌