Drop explicit unsigned from FD_SETSIZE constant

FD_SETSIZE is often used as an argument to select or compared with an
integer file descriptor.  Rather than force 3rd party software to add
explicit casts, just make it a plain (int) constant as on other
operating systems.

Previous discussion:
http://lists.freebsd.org/pipermail/freebsd-standards/2012-July/002410.html