Merge remote-tracking branch 'remotes/vivier2/tags/linux-user-for-2.13-pull-request' into staging

This pull request includes:
- fixes for some comments
- netlink update and fix
- rework/cleanup fo socket.h,
  including fixes for SPARC part.

# gpg: Signature made Fri 25 May 2018 09:16:21 BST
# gpg:                using RSA key F30C38BD3F2FBE3C
# gpg: Good signature from "Laurent Vivier <lvivier@redhat.com>"
# gpg:                 aka "Laurent Vivier <laurent@vivier.eu>"
# gpg:                 aka "Laurent Vivier (Red Hat) <lvivier@redhat.com>"
# Primary key fingerprint: CD2F 75DD C8E3 A4DC 2E4F  5173 F30C 38BD 3F2F BE3C

* remotes/vivier2/tags/linux-user-for-2.13-pull-request:
  gdbstub: Clarify what gdb_handlesig() is doing
  linux-user: define TARGET_SO_REUSEPORT
  linux-user: copy sparc/sockbits.h definitions from linux
  linux-user: update ARCH_HAS_SOCKET_TYPES use
  linux-user: move ppc socket.h definitions to ppc/sockbits.h
  linux-user: move socket.h generic definitions to generic/sockbits.h
  linux-user: move sparc/sparc64 socket.h definitions to sparc/sockbits.h
  linux-user: move alpha socket.h definitions to alpha/sockbits.h
  linux-user: move mips socket.h definitions to mips/sockbits.h
  linux-user: Fix payload size logic in host_to_target_cmsg()
  linux-user: update comments to point to tcg_exec_init()
  linux-user: update netlink emulation
  linux-user: Assert on bad type in thunk_type_align() and thunk_type_size()

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