MINOR: fd: move the hap_fd_{clr,set,isset} functions to fd.h

MINOR: fd: move the hap_fd_{clr,set,isset} functions to fd.h

These functions were created for poll() in 1.5-dev18 (commit 80da05a4) to
replace the previous FD_{CLR,SET,ISSET} that were shared with select()
because some libcs enforce a limit on FD_SET. But FD_SET doesn't seem
to be universally MT-safe, requiring locks in the select() code that
are not needed in the poll code. So let's move back to the initial
situation where we used to only use bit fields, since that has been in
use since day one without a problem, and let's use these hap_fd_*
functions instead of FD_*.

This patch only moves the functions to fd.h and revives hap_fd_isset()
that was recently removed to kill an "unused" warning.