inttostr: aesthetics and improved (compile-time) safety

Define inttype_is_signed rather than inttype_is_unsigned,
since the sole use is via "#if inttype_is_signed".
* lib/imaxtostr.c (inttype_is_signed): Define this, rather than
inttype_is_unsigned.
* lib/offtostr.c (inttype_is_signed): Likewise.
* lib/uinttostr.c (inttype_is_signed): Likewise.
* lib/umaxtostr.c (inttype_is_signed): Likewise.
* lib/inttostr.c (inttostr): Use verify to cross-check the
inttype_is_signed value and the signedness of the actual type.
* modules/inttostr (Depends-on): Add verify.