util-lib: move more file I/O related calls into fileio.[ch]
util-lib: split out fd-related operations into fd-util.[ch] There are more than enough to deserve their own .c file, hence move them over.
test-fdset: add more tests add tests for the following functions: - fdset_new_array - fdset_steal_first - fdset_isempty
tests: add test for fdset_iterate
tests: add test_fdset_remove
tests: add missing return
tests: add test-fdset