add NFDBITS in sys/select.h with appropriate feature tests
add NFDBITS in sys/select.h with appropriate feature tests

the main use for this macro seems to be knowing the correct allocation
granularity for dynamic-sized fd_set objects. such usage is
non-conforming and results in undefined behavior, but it is widespread
in applications.

chardi/musl,chardi/musl,chardi/musl