v0.4.0-beta.3

Notable changes since v0.3.0:

• Support specifying netns path (slirp4netns --netns-type=path PATH TAPNAME) (#86)
• Support specifying --userns-path (#86 )
• Vendor https://gitlab.freedesktop.org/slirp/libslirp (QEMU v4.1+) (#94)
• Bring up loopback device when --configure is specified (#99)
• Support sandboxing by creating a mount namespace and dropping capabilities except CAP_NET_BIND_SERVICE (#105)
• Fix heap overflow (CVE-2019-14378) (#127, https://gitlab.freedesktop.org/slirp/libslirp/commit/126c04acbabd7ad32c2b018fe10dfac2a3bc1210)

New build dependency: libcap-devel (libcap-dev)