linux-user: improve fake /proc/self/stat making `ps` not segfault.

linux-user: improve fake /proc/self/stat making `ps` not segfault.

With the current fake /proc/self/stat implementation `ps` is
segfaulting because it expects to read PID and argv[0] as first and
second field respectively, with the latter being enclosed between
backets.

Reproducing is as easy as running: `ps` inside qemu-user chroot
with /proc mounted.

Signed-off-by: Fabio Erculiani <lxnay@sabayon.org>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>