More accurate vi(1) word navigation in copy mode and on the status line.

This changes the meaning of the word-separators option - setting it to
the empty string is equivalent to the previous behavior. From Will Noble
in GitHub issue 2693.

options-table.c

@@ -755,7 +755,11 @@ const struct options_table_entry options_table[] = {
 	{ .name = "word-separators",
 	  .type = OPTIONS_TABLE_STRING,
 	  .scope = OPTIONS_TABLE_SESSION,
-	  .default_str = " ",
+	  /*
+	   * The set of non-alphanumeric printable ASCII characters minus the
+	   * underscore.
+	   */
+	  .default_str = "!\"#$%&'()*+,-./:;<=>?@[\\]^`{|}~",
 	  .text = "Characters considered to separate words."
 	},