rewrite check-bluepill-procs to make monitored applications configurable

This rewrite adds an option for specifying the bluepill-managed applications that should be monitored. If no applications are specified in the check options, it parses a list of currently-loaded applications out of bluepill and monitors all of them (same as legacy behavior).

We also significantly improve the check's parsing of bluepill's output - bluepill's behavior and exit codes differ based on whether it has one or multiple managed applications loaded.

This rewrite is backwards-compatible with the previous version.