commandserver: get around ETIMEDOUT raised by selectors2 selector.select() should exits with an empty event list on timed out, but selectors2 raises OSError if timeout expires while recovering from EINTR. Spotted while debugging new chg feature.

selectors2: backport minimal fix of timeout handling from 2.0.1 The original code would raise TypeError since OSError() doesn't support keyword arguments. We can't simply import the selectors 2.0.1, which still spawns "uname -p" through platform.system(). We could switch to the unreleased version, but I decided to not right now to minimize the change.

tests: sniff for libfuzzer actually being available in test-fuzz-targets.t When I upgraded the FreeBSD buildbot to 11.2 it seems we picked up clang6, but the default clang on FreeBSD doesn't include libfuzzer. I can't find a way to sniff for libfuzzer without running a compile, so here we are. Differential Revision: https://phab.mercurial-scm.org/D5270

tests: sniff for /usr/local/bin/gmake and use it in test-fuzz-targets.t This isn't as robust as it probably should be, but for now it'll get the job done on the buildbots. Differential Revision: https://phab.mercurial-scm.org/D5269