utils: Don't use directory enumerator to close open FDs in closefrom()

utils: Don't use directory enumerator to close open FDs in closefrom()

Calling malloc() after fork() is potentially unsafe, so we should avoid
it if possible.  opendir() will still require an allocation but that's
less than the variant using the enumerator wrapper, thus, decreasing
the conflict potential.  This way we can also avoid closing the
FD for the enumerated directory itself.

References #990.