Avoid page_set_flags() assert in qemu-user host page protection code

V2 that uses endaddr = end-of-guest-address-space if !h2g_valid(endaddr)
after I found out that indeed works; and also disables the FreeBSD 6.x
/compat/linux/proc/self/maps fallback because it can return partial lines
if (at least I think that's the reason) the mappings change between
subsequent read() calls.

Signed-off-by: Juergen Lock <nox@jelal.kn-bremen.de>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
