fix: dev: restore reuseport to yes by default on supported platforms

Changes introduced by 72862c2a moved the
default configuration from within `bin/named` to a central place
`bin/includes`.

The default configuration is conditioned by several compile-time macro.
While for most of them it's fine because they are defined in the global
`config.h` file included by default to all binaries (by meson), one
specific is not defined here. `HAVE_SO_REUSEPORT_LB` was defined in
`lib/isc/include/isc/netmgr.h` which is of course not included in
`bin/includes/defaultconfig.h`.

As a result, reuseport was disabled for all platform by default, even
the supported ones. This fixes the problem by checking if reuseport is
available on the platform from meson `config.h` generation directly,
which makes `HAVE_SO_REUSEPORT_LB` available everywhere.

Merge branch 'colin/fix-reuseport-default' into 'main'

See merge request isc-projects/bind9!11180