Fix backend_flag* parameter shown twice while executing "pgpool show all".

Fix backend_flag* parameter shown twice while executing "pgpool show all".

There are two entries of "backend_flag" for "ALLOW_TO_FAILOVER" and
"ALWAYS_PRIMARY" in the config variable struct. This is mostly ok but
"pgpool show all" command displayed both backend_flag entries, which
looks redundant. The reason for this is, report_all_variables() shows
grouped variables first then other variables except already shown as
grouped variables.  Unfortunately build_variable groups() is not smart
enough to build grouped variable data: it only registers the first
backend_flag entry and leaves the second entry. Since the second entry
is not a grouped variable, backend_flag is shown firstly as a grouped
variable and then is show as a non grouped variable in
report_all_variables(). To fix this, mark that the second variable is
also a grouped variable (the flag is set by
build_config_variables()). See bug 728 for the report of the problem.

Also add/fix comments.