fix Unix build with DEF_PAGER defined

Eliminate a couple of compile warnings produced when DEF_PAGER is
defined:  unixmain.c: g.catmore=DEF_PAGER; wintty.c: fd=open(...).

Override its use when DLB is also defined since an external pager
could access 'license' but not 'history', 'opthelp', &c when those
are in the dlb container file.

In the commented out value for DEF_PAGER, show a viable value for
the default configuration these days.

include/decl.h

@@ -838,7 +838,7 @@ struct instance_globals {
     int locknum; /* max num of simultaneous users */
 #endif
 #ifdef DEF_PAGER
-    char *catmore; /* default pager */
+    const char *catmore; /* external pager; from getenv() or DEF_PAGER */
 #endif
 #ifdef MICRO
     char levels[PATHLEN]; /* where levels are */