libslirp: update to 4.6.1

## [4.6.1] - 2021-06-18

### Fixed

 - Fix DHCP regression introduced in 4.6.0.

## [4.6.0] - 2021-06-14

### Added

 - mbuf: Add debugging helpers for allocation.

### Changed

 -  Revert "Set macOS deployment target to macOS 10.4".

### Fixed

 - mtod()-related buffer overflows (CVE-2021-3592, CVE-2021-3593,
   CVE-2021-3594, CVE-2021-3595).
 - poll_fd: add missing fd registration for UDP and ICMP
 - ncsi: make ncsi_calculate_checksum work with unaligned data.
 - Various typos and doc fixes.

## [4.5.0] - 2021-05-18

### Added

 - IPv6 forwarding.
 - slirp_neighbor_info() to dump the ARP/NDP tables.

### Changed

 - Lazy guest address resolution for IPv6.
 - Improve signal handling when spawning a child.
 - Set macOS deployment target to macOS 10.4.
 - slirp_add_hostfwd: Ensure all error paths set errno.
 - More API documentation.

### Fixed

 - Assertion failure on unspecified IPv6 address.
 - Disable polling for PRI on MacOS, fixing some closing streams issues.
 - Various memory leak fixes on fastq/batchq.
 - Memory leak on IPv6 fast-send.
 - Slow socket response on Windows.
 - Misc build and code cleanups.

## [4.4.0] - 2020-12-02

### Added

 - udp, udp6, icmp: handle TTL value.
 - Enable forwarding ICMP errors.
 - Add DNS resolving for iOS.

### Changed

 - Improve meson subproject() support.
 - Removed Makefile-based build system.

### Fixed

 - socket: consume empty packets.
 - check pkt_len before reading protocol header (CVE-2020-29129).
 - ip_stripoptions use memmove (fixes undefined behaviour).
 - various Coverity-related changes/fixes.


(nia)