extendif: increment RETLEN var as well as extending RET

Stupid bug causing realloc to be called all the time in case WANTLEN >
RETLEN, since RETLEN was never updated.  (It seems we just never went
beyond DESTSZ == 32, but nevertheless...)
router_route_intern: don't make any_of smart

Always send metrics to the server the hash would send it to, regardless
if it's down, since otherwise once a server goes down, it will never
receive (new) metrics any more, and therefore never come up again,
because there's simply nothing to attempt to send.  Instead rely on the
servers in the any_of pool helping each other out.
server: don't sleep twice if failed and nothing to do
server: allow hostnames or ipv6 addresses

Use first address from getaddrinfo() such that we can use real hostnames
or IPv6 addresses.
router_readconfig: rework host:port parsing

Allow for hostnames, IPv4 and IPv6 addresses, optionally bracketed to
avoid the last part of IPv6 addresses wrongly to be interpreted as port
number.
server_new_intern: spit out error message from getaddrinfo

Since getaddrinfo doesn't use errno, spit out a hint on what's wrong,
even though that might look messy.  A hint is better than nothing.
server_shutdown: cleanup saddr