Update procfs to 0.15.1 and fix compilation errors

[pstibrany]

This PR updates procfs to 0.15.1, and fixes compilation errors due to breaking change in https://github.com/prometheus/procfs/pull/623.

Another change that caused compilation problems was https://github.com/prometheus/procfs/pull/619 (issue https://github.com/prometheus/procfs/issues/450). This introduced multiple transport stats per NFS mount. ~This PR keeps reporting last stats only, which I believe was the case before https://github.com/prometheus/procfs/pull/619 was merged. Alternative would be to export all transport stats, or perhaps aggregate values together (not sure if that makes sense).~

Update: This PR now reports transport stats for each local port individually.

Replaces https://github.com/prometheus/node_exporter/pull/3037.

[pstibrany]

Looks like there are more issues to fix.

[pstibrany]

Looks like there are more issues to fix.

There was problem caused by change in https://github.com/prometheus/procfs/pull/619. PTAL at my proposed workaround (see description).

[discordianfish]

This needs rebasing

[SuperQ]

This was fixed in https://github.com/prometheus/node_exporter/pull/3059