BSD: Use byteswap.h and endian.h defined macos when present

BSD: Use byteswap.h and endian.h defined macos when present

Newer versions of FreeBSD will have a glibc compatible
byteswap.h. Currently, this file assumes that FreeBSD requires
bswap{16,32,64} to always be defined. This isn't the case when we're
using the byteswap.h file (which is enabled by HAVE_BYTESWAP_H
define). Assume that if byteswap.h and endian.h are present, then the
proper macros are defined.  Otherwise, assume that sys/endian.h is
present and paper over the small differences between the BSDs for this
file.

Signed-off-by: Warner Losh <imp@bsdimp.com>