split get_make_jobserver_status into two functions

split get_make_jobserver_status into two functions

I disliked the complexity of the return values, and the boilerplate
code that followed the two calls to the function, to clean/unset
MAKEFLAGS. To solve both, I refactored it into two functions, one simply
tests to see if a jobserver is specified but unavailable, while the other
cleans/unsets MAKEFLAGS.

This loses the ability to pull the jobs-N count out of MAKEFLAGS,
but that was not currently used.