Merge remote-tracking branch 'remotes/riku/linux-user-for-upstream' into staging

* remotes/riku/linux-user-for-upstream:
  linux-user: Implement capget, capset
  linux-user: Don't allow guest to block SIGSEGV
  signal: added a wrapper for sigprocmask function
  linux-user: Don't reserve space for commpage for AArch64
  linux-user: implement F_[GS]ETOWN_EX
  linux-user: Don't return uninitialized value for atomic_barrier syscall
  linux-user/signal.c: Correct error path for AArch64 do_rt_sigreturn

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>