main/nsd: --enable-root-server at configure time

Sometimes it's handy to configure a catch-all zone on nsd.

However, nsd cannot be configured as root server at runtime. The only way to allow such usage is to configure at compile-time.

This MR enable the corresponding flag.

source code reference:

• https://github.com/NLnetLabs/nsd/blob/5e42dc839f6f43c7dea27a9daca4f0d9c370caa3/configure.ac#L882-L889
• https://github.com/NLnetLabs/nsd/blob/42552a508cb426e997e79a3ed20f407b4af0b747/nsd-checkconf.c#L667-L673
• https://github.com/NLnetLabs/nsd/blob/42552a508cb426e997e79a3ed20f407b4af0b747/zonec.c#L1615-L1621