Allow "dbname" from connection string to be overridden in PQconnectDBParams

If the "dbname" attribute in PQconnectDBParams contained a connection string
or URI (and expand_dbname = TRUE), the database name from the connection
string could not be overridden by a subsequent "dbname" keyword in the
array. That was not intentional; all other options can be overridden.
Furthermore, any subsequent "dbname" caused the connection string from the
first dbname value to be processed again, overriding any values for the same
options that were given between the connection string and the second dbname
option.

In the passing, clarify in the docs that only the first dbname option in the
array is parsed as a connection string.

Alex Shulgin. Backpatch to all supported versions.

Author: hlinnaka

src/interfaces/libpq/fe-connect.c

@@ -4460,9 +4460,9 @@ conninfo_array_parse(const char **keywords, const char **values,
 			}
 
 			/*
-			 * If we are on the dbname parameter, and we have a parsed
-			 * conninfo string, copy those parameters across, overriding any
-			 * existing previous settings
+			 * If we are on the first dbname parameter, and we have a parsed
+			 * connection string, copy those parameters across, overriding any
+			 * existing previous settings.
 			 */
 			if (strcmp(pname, "dbname") == 0 && str_options)
 			{
@@ -4494,6 +4494,12 @@ conninfo_array_parse(const char **keywords, const char **values,
 						}
 					}
 				}
+				/*
+				 * Forget the parsed connection string, so that any subsequent
+				 * dbname parameters will not be expanded.
+				 */
+				PQconninfoFree(str_options);
+				str_options = NULL;
 			}
 			else
 			{