vis: support rudimentary help search using :help pattern

The argument needs to be a valid regular expression. It currently
clobbers the last used search term and hence allows repeated searches
using `n` and `N` respectively.

sam.c

@@ -215,7 +215,7 @@ static const CommandDef cmds[] = {
 		CMD_FORCE|CMD_ONCE|CMD_ADDRESS_NONE|CMD_DESTRUCTIVE, NULL, cmd_bdelete
 	}, {
 		"help",         "Show this help",
-		CMD_ONCE|CMD_ADDRESS_NONE, NULL, cmd_help
+		CMD_ARGV|CMD_ONCE|CMD_ADDRESS_NONE, NULL, cmd_help
 	}, {
 		"map",          "Map key binding `:map <mode> <lhs> <rhs>`",
 		CMD_ARGV|CMD_FORCE|CMD_ONCE|CMD_ADDRESS_NONE, NULL, cmd_map

vis-cmds.c

@@ -753,6 +753,9 @@ static bool cmd_help(Vis *vis, Win *win, Command *cmd, const char *argv[], Curso
 		text_appendf(txt, "  %-32s\t%s\n", configs[i].name, configs[i].enabled ? "yes" : "no");
 
 	text_save(txt, NULL);
+
+	if (argv[1])
+		vis_motion(vis, VIS_MOVE_SEARCH_FORWARD, argv[1]);
 	return true;
 }