This is done by performing one synchronous query for carrier state at
the start of the program; after that, we just monitor the nlsocket for
carrier state changes and update the cached state accordingly.
The benefit is that ifchd needs to do a lot less work and this should
reduce the CPU cycle consumption; prior to this commit, the CPU time
ends up being a few CPU-minutes per month.
'(c)' may not be a valid substitute for 'Copyright' in some legal
domains/interpretations. So be safe, since I obviously am asserting
copyright on my legal work.
This change paves the way for allowing ifch to notify the core ndhc
about failures. It would be far too difficult to reason about the
state machine if the requests to ifch were asynchronous.
Currently ndhc assumes that ifch requests never fail, but this
is not always true because of eg, rfkill.
The pipes wouldn't do this job anymore because they were unused and thus
never performed writes that would generate SIGPIPEs, so the pipes are
removed, too.
