libnpupnp: update to 6.3.0

libnpupnp: update to 6.3.0

Changes since 6.2.1:
- Build without expat by default (switch to picoxml)
- Fix build errors with -Dipv6=false option (issue #142)
- Check size of POST upload data and response data against
  g_maxContentLength; change default from 16k to 2MB
- Use IPv6 socket when looking for an available port with IPv6 enabled
- Fix MHD start error handling to not delete minisocket array prematurely
- Remove code assuming different v4/v6 listening ports
- Add per-subsystem debug logging control via environment variable
- SSDP code cleanups and header removal
- Fix win32 compile issue (setsockopt args)
- Avoid leaking private headers to consumers via meson build

Signed-off-by: Alexandru Ardelean <redacted>