Introduce output_primary_name function

Currently simply returns output->name, but this will make it easier to
change how output names are stored in the following commits.

Also replace reading output->name with invocations of
output_primary_name. Code which writes output->name is unchanged. Done
using a mostly mechanical replacement of output->name to
output_primary_name(output).

include/output.h

@@ -24,6 +24,12 @@ Con *output_get_content(Con *output);
  */
 Output *get_output_from_string(Output *current_output, const char *output_str);
 
+/**
+ * Retrieves the primary name of an output.
+ *
+ */
+char *output_primary_name(Output *output);
+
 /**
  * Returns the output for the given con.
  *