linux-user: increment MAX_ARG_PAGES

linux-user: increment MAX_ARG_PAGES

There's a error When doing something like that :
find / -type f -print0 | xargs -0 echo

[ done in a arm chroot with qemu-arm and linux binfmt stuff or with
find / -type f -print0 | qemu-arm -L <path> <path>/usr/bin/xargs -0
echo ]

Doing this outsite qemu is fine. The problem was the huge number of
parameters. Increasing MAX_ARG_PAGES is fixing that.

While I was at it, I've modified linux-user/main.c to report error code
of loader_exec. It helps to debug/know what's wrong.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>