Merge with gnulib, pacifying GCC 7

This incorporates:
2017-05-16 manywarnings: update for GCC 7
2017-05-15 sys_select: Avoid "was expanded before it was required"
* configure.ac (nw): Suppress GCC 7’s new -Wduplicated-branches and
-Wformat-overflow=2 options, due to too many false alarms.
* doc/misc/texinfo.tex, lib/strftime.c, m4/manywarnings.m4:
Copy from gnulib.
* m4/gnulib-comp.m4: Regenerate.
* src/coding.c (decode_coding_iso_2022):
Fix bug uncovered by -Wimplicit-fallthrough.
* src/conf_post.h (FALLTHROUGH): New macro.
Use it to mark all switch cases that fall through.
* src/editfns.c (styled_format): Use !, not ~, on bool.
* src/gtkutil.c (xg_check_special_colors):
When using sprintf, don’t trust Gtk to output colors in [0, 1] range.
(xg_update_scrollbar_pos): Avoid use of possibly-uninitialized bool;
this bug was actually caught by Clang.
* src/search.c (boyer_moore):
Tell GCC that CHAR_BASE, if nonzero, must be a non-ASCII character.
* src/xterm.c (x_draw_glyphless_glyph_string_foreground):
Tell GCC that glyph->u.glyphless.ch must be a character.

src/coding.c

@@ -3611,7 +3611,7 @@ decode_coding_iso_2022 (struct coding_system *coding)
 	      || CODING_ISO_FLAGS (coding) & CODING_ISO_FLAG_SEVEN_BITS)
 	    goto invalid_code;
 	  /* This is a graphic character, we fall down ... */
-
+	  FALLTHROUGH;
 	case ISO_graphic_plane_1:
 	  if (charset_id_1 < 0)
 	    goto invalid_code;
@@ -3646,6 +3646,7 @@ decode_coding_iso_2022 (struct coding_system *coding)
 	case ISO_single_shift_2_7:
 	  if (! (CODING_ISO_FLAGS (coding) & CODING_ISO_FLAG_SEVEN_BITS))
 	    goto invalid_code;
+	  FALLTHROUGH;
 	case ISO_single_shift_2:
 	  if (! (CODING_ISO_FLAGS (coding) & CODING_ISO_FLAG_SINGLE_SHIFT))
 	    goto invalid_code;
@@ -3797,6 +3798,7 @@ decode_coding_iso_2022 (struct coding_system *coding)
 		{
 		case ']':	/* end of the current direction */
 		  coding->mode &= ~CODING_MODE_DIRECTION;
+		  break;
 
 		case '0':	/* end of the current direction */
 		case '1':	/* start of left-to-right direction */