startup: show the correct number of lines when opening multiple files

When switching to a different buffer, don't just show its name but
also the number of lines it contains.  This is useful extra info.

Then use this same message when at startup multiple files are opened
and (after reading them all) we switch back to the first buffer.

(This loses, when multiple files are opened, the information about
format conversion that nano still shows when a single file is opened,
but... this bug has shown that people don't really look at this line
anyway, so... let it be.  The info can still be seen when writing out
the file with ^O.)

This addresses https://savannah.gnu.org/bugs/?54047.

src/files.c

@@ -585,6 +585,17 @@ void prepare_for_display(void)
 }
 
 #ifdef ENABLE_MULTIBUFFER
+/* Show name of current buffer and its number of lines on the status bar. */
+void mention_name_and_linecount(void)
+{
+	size_t count = openfile->filebot->lineno -
+						(openfile->filebot->data[0] == '\0' ? 1 : 0);
+
+	statusline(HUSH, P_("%s -- %zu line", "%s -- %zu lines", count),
+						openfile->filename[0] == '\0' ?
+						_("New Buffer") : tail(openfile->filename), count);
+}
+
 /* Switch to a neighbouring file buffer; to the next if to_next is TRUE;
  * otherwise, to the previous one. */
 void switch_to_adjacent_buffer(bool to_next)
@@ -615,10 +626,8 @@ void switch_to_adjacent_buffer(bool to_next)
 	/* Ensure that the main loop will redraw the help lines. */
 	currmenu = MMOST;
 
-	/* Indicate the switch on the statusbar. */
-	statusline(HUSH, _("Switched to %s"),
-						((openfile->filename[0] == '\0') ?
-						_("New Buffer") : openfile->filename));
+	/* Indicate on the status bar where we switched to. */
+	mention_name_and_linecount();
 
 #ifdef DEBUG
 	dump_filestruct(openfile->current);