readutmp: Add option to make use of wtmpdb.

readutmp: Add option to make use of wtmpdb.

* m4/wtmpdb.m4: New file.
* m4/readutmp.m4 (gl_READUTMP): Invoke gl_WTMPDB_CHOICE. Define
READUTMP_USE_WTMPDB. Take it into account for READUTMP_LIB.
* lib/readutmp.h (WTMPDB_FILE): New macro.
* lib/readutmp.c: Include <wtmpdb.h>.
(wtmpdb_time_to_timespec): New function.
(struct wtmpdb_locals): New type.
(wtmpdb_callback, read_utmp_from_wtmpdb): New functions.
(read_utmp): For WTMP_FILE, conditionally use read_utmp_from_wtmpdb.
* modules/readutmp (Files): Add m4/wtmpdb.m4.
(Depends-on): Add str_endswith.
* DEPENDENCIES: Mention libwtmpdb.