sm-notify: Failed DNS lookups should be retried

sm-notify: Failed DNS lookups should be retried

Currently, if getaddrinfo(3) fails when trying to resolve a hostname,
sm-notify gives up immediately on that host.  If sm-notify is started
before network service is available on a system, that means it quits
without notifying anyone.  Or, if DNS service isn't available due to
a network partition or because the DNS server crashed, sm-notify will
simply remove all of its callback files and exit.

Really, sm-notify should try harder.  We know that the hostnames
passed in to notify_host() have already been vetted by statd, which
won't monitor a hostname that it can't resolve.  So it's likely that
any DNS failure we meet here is a temporary condition.  If it isn't,
then sm-notify will stop trying to notify that host in 15 minutes
anyway.

[ The host's file is left in /var/lib/nfs/sm.bak in this case, but
  sm.bak is not read again until the next time sm-notify runs. ]

sm-notify already has retry logic for handling RPC timeouts.  We can
co-opt that to drive DNS resolution retries.

We also add AI_ADDRCONFIG because on systems whose network startup is
handled by NetworkManager, there appears to be a bug that causes
processes that started calling getaddinfo(3) before the network came
up to continue getting EAI_AGAIN even after the network is fully
operating.

As I understand it, legacy glibc (before AI_ADDRCONFIG was exposed in
headers) sets AI_ADDRCONFIG by default, although I haven't checked
this.  In any event, pre-glibc-2.2 systems probably won't run
NetworkManager anyway, so this may not be much of a problem for them.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>