D3716: ui: add an uninterruptable context manager that can block SIGINT

[Augie Fackler]

>>>  Do we need a different strategy for Windows, or just eat the error?  (See the unsupported signal message)
>>> 
>>>  https://buildbot.mercurial-scm.org/builders/Win7%20x86_64%20hg%20tests/builds/808/steps/run-tests.py%20%28python%202.7.13%29/logs/stdio
>> 
>> I'm not sure. We could probably just not support this on windows for now, or at least disable the test on Windows?
> 
> 
>  I’m ok with disabling the test as long as it doesn’t explode on Windows without turning the experimental knobs.  It sounded like future would would depend on this, and I wasn’t sure if that landed yet.

We’ve already made some use of it in core - anything that calls strip will hit this context manager, as will the (experimental) narrow extension.