[darwin][process]: fix errno handling and library lifetime on darwin

Read errno through __error() resolved before the libc call, so that the
dlsym and allocation performed by symbol resolution cannot overwrite it
first. Apply the same to CwdWithContext, which additionally read errno
without checking for failure and could surface a stale EPERM.

Share dlopen handles process-wide instead of opening and closing them per
call, completing for libSystem what #1832 did for IOKit and CoreFoundation.

Stop populating ReadBytes/WriteBytes on darwin, which #2117 filled with the
disk figures. They mean all I/O including cache on other platforms, so
reusing the disk value there would silently change what the field means
depending on the platform.