Spawning / Forking a process on QNX Neutrino can fail

[flba-eb]

This only applies to the Tier 3 target QNX Neutrino OS versions 7.1 and before.

When a process runs fork/spawn in one thread and changes existing file descriptors in another thread at the same time, fork/spawn can fail. This seems not to be very common on applications, but happens a lot in Rusts test suites and remote-test-server.

As of #106673, Rusts standard library retries four times (as suggested by QNX' documentation), but this can be insufficient. Up to 150 retries have been observed when running the test suite.

See #106673 for more details.

cc: @gh-tr

@rustbot label +O-neutrino