linux-user: Use safe_syscall for open and openat system calls

Restart open() and openat() if signals occur before,
or during with SA_RESTART.

Signed-off-by: Timothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
Message-id: 1441497448-32489-17-git-send-email-T.E.Baldwin99@members.leeds.ac.uk
[PMM: Adjusted to follow new -1-and-set-errno safe_syscall convention]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>