string-buffer, string-buffer-reversed: Allow for better static analysis.
* lib/string-buffer.h (sb_appendvf, sb_appendf, sb_xappendvf,
sb_xappendf): Don't declare if SB_NO_APPENDF is defined.
(sb_xdupfree_c): Declare a non-NULL return value if SB_NO_APPENDF is
defined.
* lib/string-buffer-reversed.h (sbr_prependvf, sbr_prependf,
sbr_xprependvf, sbr_xprependf): Don't declare if SBR_NO_PREPENDF is
defined.
(sbr_xdupfree_c): Declare a non-NULL return value if SBR_NO_PREPENDF is
defined.