Add opt_toggle_disallowed to abort toggling a string option

before the user enters the new string.
Use it to abort -o if the input is not a pipe.

command.c

@@ -1793,6 +1793,12 @@ commands(VOID_PARAM)
  optgetname = FALSE;
  mca_opt_toggle();
  c = getcc();
+ cbuf = opt_toggle_disallowed(c);
+ if (cbuf != NULL)
+ {
+ error(cbuf, NULL_PARG);
+ break;
+ }
  goto again;
 
  case A_DISP_OPTION:

option.c

@@ -519,6 +519,24 @@ opt_prompt(o)
  return (o->odesc[0]);
 }
 
+/*
+ * If the specified option can be toggled, return NULL.
+ * Otherwise return an appropriate error message.
+ */
+ public char *
+opt_toggle_disallowed(c)
+ int c;
+{
+ switch (c)
+ {
+ case 'o':
+ if (ch_getflags() & CH_CANSEEK)
+ return "Input is not a pipe";
+ break;
+ }
+ return NULL;
+}
+
 /*
  * Return whether or not there is a string option pending;
  * that is, if the previous option was a string-valued option letter