(dflt_econfig): New.

(emp_config): Use it when argument is NULL.  This simplifies callers
except pconfig's main(), which now has to suppress reading the default
file.  Also fix callers to check the return value.  Previously, a typo
could easily start a grossly misconfigured program, with potentially
disastrous results.

include/optlist.h

@@ -37,6 +37,9 @@
 
 #include "nsc.h"
 
+/* Default econfig file */
+extern char dflt_econfig[];
+
 struct option_list {
     char *opt_key;
     int *opt_valuep;