*_startswith, *_endswith: Change return type to 'bool'.

*_startswith, *_endswith: Change return type to 'bool'.

Suggested by Paul Eggert.

* lib/string.in.h (str_startswith, str_endswith, mbs_startswith,
mbs_endswith): Change return type to 'bool'.
* lib/str_startswith.c (str_startswith): Likewise.
* lib/str_endswith.c (str_endswith): Likewise.
* lib/mbs_endswith.c (mbs_endswith): Likewise.
* modules/str_startswith (Depends-on): Add bool.
* modules/str_endswith (Depends-on): Likewise.
* modules/mbs_startswith (Depends-on): Likewise.
* modules/mbs_endswith (Depends-on): Likewise.