gopsutil/internal/common
elij d1380cba29 try to fix diskio stats on FreeBSD-10.x
FreeBSD apparently changed the magic sysctl mib values for devstats.

    --- FAIL: TestDisk_io_counters (0.00s)
    disk_test.go:39: error no such file or directory
    disk_test.go:42: ret is empty, map[]

This code uses an undocumented, but exported, go stdlib method to fetch
the sysctl by string instead of mib.
..
common.go fix typos
common_darwin.go common: rename package common to internal/common. Add ENV helper funcs.
common_freebsd.go try to fix diskio stats on FreeBSD-10.x
common_linux.go common: rename package common to internal/common. Add ENV helper funcs.
common_test.go common: rename package common to internal/common. Add ENV helper funcs.
common_unix.go process[linux,freebsd,darwin]: implements Children using pgrep.
common_windows.go common: rename package common to internal/common. Add ENV helper funcs.