[analyzer] detect `errno` misuse

It would be helpful to have an analysis which detects the incorrect usage of `errno`:
- checking after calls which do not set it
- not checking after calls which set it
- checking after a call which sets it but with another call which sets it in-between
- checking for values which are not being used by the call

The issue does not name files, tests, or an analyzer entry point. Start by locating the analyzer component responsible for errno-related checks, then define coverage for each listed misuse and add tests showing the analysis detects them without false positives.

Written by the indexing model from the issue text.