help: do not reserve space for line numbers, as they are absent

Help texts are shown without any line numbers, so the full width
of the screen should be used to display the text.

This fixes https://savannah.gnu.org/bugs/?53308.

src/help.c

@@ -146,6 +146,7 @@ void do_help(void)
 
 #ifdef ENABLE_LINENUMBERS
 	UNSET(LINE_NUMBERS);
+	editwincols = COLS;
 	margin = 0;
 #endif
 	tabsize = 8;
@@ -253,6 +254,7 @@ void do_help(void)
 
 #ifdef ENABLE_LINENUMBERS
 	margin = was_margin;
+	editwincols = COLS - margin;
 #endif
 	tabsize = was_tabsize;
 #ifdef ENABLE_COLOR