BUG/MINOR: resolvers: warn about ignored arguments on datagram nameservers

BUG/MINOR: resolvers: warn about ignored arguments on datagram nameservers

Since TCP nameservers were supported, the "nameserver" line accepts server
keywords after the address (e.g. "ssl", "verify"), which are parsed by
parse_server() for "tcp@" addresses. For datagram nameservers these
arguments are silently ignored. A typo such as forgetting the "tcp@"
prefix on "nameserver ns1 1.2.3.4:853 ssl verify none" thus results in
plain DNS being sent over UDP to the DoT port (and, since commit bc20f8c0b
("MEDIUM: resolvers: retry truncated UDP responses over TCP"), over plain
TCP for fallback queries) without any notice.

Let's emit a warning when arguments follow the address of a datagram
nameserver. A warning rather than an error is used so as not to break
existing configurations.

This patch may be backported to stable branches.