linux-user: check some parameters for some socket syscalls.

This patch is fixing following issues :

- commit 8fea36025b9d6d360ff3b78f88a84ccf221807e8 was applied to
  do_getsockname instead of do_accept.
- Some syscalls were not checking properly the memory addresses passed
  as argument
- Add check before syscalls made for cases like do_getpeername() where
  we're using the address parameter after doing the syscall
- Fix do_accept to return EINVAL instead of EFAULT when parameters
  invalid to match with linux behaviour

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