[RFE] username authentication for pmdaproc

[natoscott]

The proc metrics currently require UID-based authentication in order to allow fetch access - this is achieved using Unix-domain socket auto-authentication for most PCP tools, locally. However users also want to access proc metric values using the REST API, and this provides USERNAME based authentication attribute to pmdaproc (which it ignores currently).

A scheme similar to pmdabpftrace - where an allowed_list of usernames can be specified - would be one way to solve this (stitching this into the logic behind the -A option and have_access.

An alternative (or additional) scheme could see pmdaproc attempt to associate the USERNAME attribute with a local UID, and if successful allow access in the same way the Unix-domain socket authentication functions.