unable to collect metrics for some processes on 32bit arm

[flixr]

At some point during runtime I suddenly didn't get any metrics for some processes anymore.

After adding some more debug output (see #79) I got this:

2019/01/15 13:10:05 error getting proc ID for pid 2434: integer overflow on token 2264576000

It seems this is because ProcStat in procfs/proc_stat.go uses too small integers for some fields.
in my case it is the vsize field that has the overflow.
The vsize should be unsigned as well.

Note that this happend on armv6, so 32bit.

[ncabatoff]

I've merged in the upstream changes, thanks! I've only done smoke testing on amd64 so far, but I expect you'll be pleased to hear that I'm now using pi zeros in my home cluster, and they're running process-exporter! So you should no longer be the sole finder of armv6/32-bit issues.