Application name for pg_basebackup and friends

Hi,

The attached patch adds an -a / --appname command line switch to
pg_basebackup, pg_receivewal and pg_recvlogical.

This is useful when f.ex. pg_receivewal needs to connect as a
synchronous client (synchronous_standby_names),

  pg_receivewal -h myhost -p 5432 -S replica1 -a replica1 --synchronous
-D /wal

I'll add the patch to the CommitFest for discussion, as there is overlap
with the -d switch.

Best regards,
  Jesper