Now <utmpx.h> isn't any more useful on Android than <utmp.h> is, but it
is POSIX, and -- importantly -- we can implement it with just a header
file, so code can use it on every existing API level.
macOS does indeed only have the <utmpx.h> functions (although it does
still have the <utmp.h> header!), so potentially portable code might
want <utmpx.h> on Android. (glibc/musl both have both headers.)
Bug: https://github.com/landley/toybox/pull/213
Test: treehugger
Change-Id: Iaa88167708182009a63e2e1a15f11186b251ed02
