Previously, we were using `umask(mask = umask(0))` to fetch the current
umask in order to apply it to the desired mode, but this is broken in
the presence of threads. There is no portable way to directly fetch
umask without mutating it.

Instead, add a reimplementation of mkstemp that uses a passed-in mode,
instead of explicitly chmodding to 0600 like POSIX requires of mkstemp.

Fixes: jmgao/pore#46