Enhance debug message upon receiving startup packet.

Enhance debug message upon receiving startup packet.

While processing a startup packet, database name, user name and
application name are printed in DEBUG1, but other guc variables (if
any) were not printed. This is not helpful when studying errors like
"connection exists but startup packet length is not identical" problem
(see https://www.pgpool.net/mantisbt/view.php?id=696). With this
commit guc variables are now printed something like:

2021-03-16 10:21:32: child pid 5155: DEBUG:  reading startup packet
2021-03-16 10:21:32: child pid 5155: DETAIL:  guc name: client_encoding value: UTF8