for multithreaded set*id/setrlimit, handle case where callback does not run

for multithreaded set*id/setrlimit, handle case where callback does not run

in the current version of __synccall, the callback is always run, so
failure to handle this case did not matter. however, the upcoming
overhaul of __synccall will have failure cases, in which case the
callback does not run and errno is already set. the changes being
committed now are in preparation for that.