Enable history substitution of interactive commands only when variable 'histsubst' is set.

This feature is in conflict with ! (logic inversion)
in .control language logic expressions.

src/frontend/init.c

@@ -20,6 +20,7 @@ cp_init(void)
    cp_chars[128]
    cp_maxhistlength (set to 10000 in com_history.c)
    cp_curin, cp_curout, cp_curerr (defined in streams.c)
+   cp_no_histsubst
 */
 {
     cp_vset("history", CP_NUM, &cp_maxhistlength);
@@ -28,6 +29,10 @@ cp_init(void)
     cp_curout = stdout;
     cp_curerr = stderr;
 
+    /* Enable history substitution */
+    if (cp_getvar("histsubst", CP_BOOL, NULL, 0))
+        cp_no_histsubst = FALSE;
+
     /* io redirection in streams.c:
        cp_in set to cp_curin etc. */
     cp_ioreset();

src/frontend/parser/cshpar.c

@@ -37,8 +37,8 @@ Author: 1985 Wayne A. Christopher, U. C. Berkeley CAD Group
 #include <sys/wait.h>
 #endif
 
-
-bool cp_no_histsubst = FALSE; /* perform history substitution by default */
+/* perform history substitution only when variable 'histsubst' is set */
+bool cp_no_histsubst = TRUE;
 
 /* Things go as follows:
  * (1) Read the line and do some initial quoting (by setting the 8th bit),