don't define _XOPEN_SOURCE for NetBSD

on NetBSD, defining _XOPEN_SOURCE hides extensions
like inet_aton, strcasecmp, etc.
rename popcount to popcount_binary to avoid a conflict with NetBSD libc

NetBSD-current's libc has a function named popcount.
hiding these extensions using feature macros is not possible because
redis uses other extensions covered by the same feature macro.
eg. inet_aton
use nanosleep instead of usleep

SUSv3 says that:
	The useconds argument shall be less than one million. If the value of
	useconds is 0, then the call has no effect.
and actually NetBSD's implementation rejects such a value with EINVAL.
use nanosleep which has no such a limitation instead.
don't assume time_t == long

time_t is always 64bit on recent versions of NetBSD.
Merge pull request #1111 from yamt/netbsd3

netbsd support