Fixed order of includes. <sys/cdefs.h> must be included before
<machine/ansi.h> for the definition of __signed, unless the compiler
is gcc.

Moved the declaration of socklen_t up together with the declaration of
size_t, and removed low-quality comment about this declaration. Declaring
socklen_t in this file is normal in POSIX-1.200x, unlike declaring size_t.