haproxy: Update HAProxy to v1.8.4 (+patches)

haproxy: Update HAProxy to v1.8.4 (+patches)
- Update haproxy download URL and hash
- Update the haproxy homepage
- Add libatomic to the dependencies as 1.8 needs it
- Make USE_REGPARM an x86-only option as this fixes many warnings and does not do much on non-x86 platforms
- Add USE_GETADDRINFO=1 to use getaddrinfo() to resolve IPv6 host names
- Add USE_TFO=1 to enable TCP fast open
- Unbreak CFLAGS, LD and LDFLAGS by adding the missing backslash after $(ADDON)
- Unbreak IGNOREGIT=1 option (typo)
- Rework LDFLAGS and add libatomic
- Add MEDIUM+ patches (see https://www.haproxy.org/bugs/bugs-1.8.4.html)

Signed-off-by: Christian Lachner <gladiac@gmail.com>