Eliminate spurious type-pun warning during HP-UX GCC build.

While the "fd_set" type is defined in /usr/include/sys/_fd_macros.h as a
struct of an array of "long" values in accordance with the XPG4 standard's
requirements, the macros for the FD operations "pretend it is an array of
int32_t's so the binary layout is the same for both Narrow and Wide
processes," as described in _fd_macros.h. In the FD_SET, FD_CLR, and
FD_ISSET macros at line 101, the result is cast to an "__fd_mask *" type,
which is defined as int32_t at _fd_macros.h:82.

This conflict between the "long fds_bits[]" array in the XPG4-compliant
fd_set structure, and the cast to an int32_t - not long - pointer in the
macros, causes a type-pun warning if -Wstrict-aliasing is enabled.
The warning is merely a side effect of HP-UX working as designed,
so it can be ignored.
Merge pull request #287 from mvpel/hp_fdset_type_pun

Eliminate spurious type-pun warning during HP-UX GCC build.