[process][posix] Fix #1049 check if procfs is mounted before checking if pid exists there

[Lomanic]

[process][posix] Fix #1049 check if procfs is mounted before checking if pid exists there

Benchmark before this change (process.NewProcess() calls process.PidExistsWithContext()
internally)

go test -bench=BenchmarkNewProcess github.com/shirou/gopsutil/process
goos: linux
goarch: amd64
pkg: github.com/shirou/gopsutil/process
BenchmarkNewProcess-4              14722             78751 ns/op
PASS
ok      github.com/shirou/gopsutil/process      3.685s

Benchmark with this change applied

go test -bench=BenchmarkNewProcess github.com/shirou/gopsutil/process
goos: linux
goarch: amd64
pkg: github.com/shirou/gopsutil/process
BenchmarkNewProcess-4              14835             80180 ns/op
PASS
ok      github.com/shirou/gopsutil/process      3.761s

[shirou]

I have confirmed this works on my FreeBSD which I unmount the procfs. Thank you so much!