This replaces some old Chromium code that tries to minimally disentangle
an arbitrary file descriptor mapping with simpler algorithm, for several
reasons:
1. Do something appropriate when a file descriptor is mapped to the same
fd number in the child; currently they're ignored, which means they'll
be closed if they were close-on-exec. This implementation duplicates
the fd twice in that case, which seems to be uncommon in practice; this
isn't maximally efficient but avoids special-case code.
2. Make this more generally applicable; the previous design is
specialized for arbitrary code running between fork and exec, but we
also want to use this on OS X with posix_spawn, which exposes a very
limited set of operations.
3. Avoid the use of C++ standard library iterators in async signal safe
code; the Chromium developers mention that this is a potential problem in
some debugging implementations that take locks.
4. In general the algorithm is simpler and should be more "obviously
correct"; more concretely, it should get complete coverage just by being
run normally in a debug build.
As a convenient side benefit, CloseSuperfluousFds now takes an arbitrary
predicate for which fds to leave open, which means it can be used in
other code that needs it without creating a fake fd mapping.
MozReview-Commit-ID: EoiRttrbrKL
