Mark application_name as GUC_REPORT so that the value will be reported back

Mark application_name as GUC_REPORT so that the value will be reported back
to the client by the server.  This might seem pretty pointless but apparently
it will help pgbouncer, and perhaps other connection poolers.  Anyway it's
practically free to do so for the normal use-case where appname is only set
in the startup packet --- we're just adding a few more bytes to the initial
ParameterStatus response packet.  Per comments from Marko Kreen.