dns: Allow the dns resolver to retrieve a server set

Allow the DNS resolver to retrieve a set of servers and their associated
addresses, ports, preference and weight ratings.

In terms of communication with userspace, "srv=1" is added to the callout
string (the '1' indicating the maximum data version supported by the
kernel) to ask the userspace side for this.

If the userspace side doesn't recognise it, it will ignore the option and
return the usual text address list.

If the userspace side does recognise it, it will return some binary data
that begins with a zero byte that would cause the string parsers to give an
error. The second byte contains the version of the data in the blob (this
may be between 1 and the version specified in the callout data). The
remainder of the payload is version-specific.

In version 1, the payload looks like (note that this is packed):

u8 Non-string marker (ie. 0)
u8 Content (0 => Server list)
u8 Version (ie. 1)
u8 Source (eg. DNS_RECORD_FROM_DNS_SRV)
u8 Status (eg. DNS_LOOKUP_GOOD)
u8 Number of servers
foreach-server {
u16 Name length (LE)
u16 Priority (as per SRV record) (LE)
u16 Weight (as per SRV record) (LE)
u16 Port (LE)
u8 Source (eg. DNS_RECORD_FROM_NSS)
u8 Status (eg. DNS_LOOKUP_GOT_NOT_FOUND)
u8 Protocol (eg. DNS_SERVER_PROTOCOL_UDP)
u8 Number of addresses
char[] Name (not NUL-terminated)
foreach-address {
u8 Family (AF_INET{,6})
union {
u8[4] ipv4_addr
u8[16] ipv6_addr
}
}
}

This can then be used to fetch a whole cell's VL-server configuration for
AFS, for example.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>