readutmp, boot-time: On Haiku, return the boot time.
* m4/readutmp.m4 (gl_PREREQ_READUTMP_H): Test whether <OS.h> exists.
* lib/boot-time-aux.h (get_haiku_boot_time,
get_haiku_boot_time_final_fallback): New functions.
* lib/readutmp.c: Include <OS.h>.
(read_utmp_from_file): If opening UTMP_FILE fails, continue processing
instead of failing. Invoke get_haiku_boot_time and
get_haiku_boot_time_final_fallback.
* lib/boot-time.c: Include <OS.h>.
(get_boot_time_uncached): Invoke get_haiku_boot_time and
get_haiku_boot_time_final_fallback.