Merge remote-tracking branch 'remotes/elmarco/tags/libslirp-pull-request' into staging

Update libslirp

Hi,

v4:
 - drop subproject patch
 - fix OSX linking issue

v3:
 - rebased
 - (checked compilation with P. Maydell extra-cflags reported failure & gitlab CI)

v2:
 - fix unused variables on macos
 - fork_exec_child_setup: improve signal handling

# gpg: Signature made Tue 03 Aug 2021 15:17:39 BST
# gpg:                using RSA key 87A9BD933F87C606D276F62DDAE8E10975969CE5
# gpg:                issuer "marcandre.lureau@redhat.com"
# gpg: Good signature from "Marc-André Lureau <marcandre.lureau@redhat.com>" [full]
# gpg:                 aka "Marc-André Lureau <marcandre.lureau@gmail.com>" [full]
# Primary key fingerprint: 87A9 BD93 3F87 C606 D276  F62D DAE8 E109 7596 9CE5

* remotes/elmarco/tags/libslirp-pull-request:
  Update libslirp to v4.6.1

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