readutmp, boot-time: Fix compilation error on old Android. Reported by Po Lu in <https://lists.gnu.org/archive/html/bug-gnulib/2023-08/msg00108.html>. * lib/readutmp.c (endutent): New fallback declaration, for Android. (getutent): Remove Ultrix workaround from 2000-04-05. * lib/boot-time.c: Likewise. * m4/readutmp.m4 (gl_PREREQ_READUTMP_H): Test whether endutent is declared, not getutent. * doc/glibc-functions/endutent.texi: Mention the Android bug.