BUG/MINOR: server: explicitly set "none" init-addr for dynamic servers

BUG/MINOR: server: explicitly set "none" init-addr for dynamic servers

Define srv.init_addr_methods to SRV_IADDR_NONE on 'add server' CLI
handler. This explicitly states that no resolution will be made on the
server creation.

This is not a real bug as the default value (SRV_IADDR_END) has the same
effect in practice. However the intent is clearer and prevent to use the
default "libc,last" by mistake which cannot execute on runtime (blocking
call + file access via gethostbyname/getaddrinfo).

The doc is also updated to reflect this limitation.

This should be backported up to 2.4.