main/busybox: build dynamic busybox with utmps

Enable support for utmp and wtmp in certain busybox applets (e.g. login,
su) and add who(1) and last(1) applets. This adds +8 kiB on x86_64.

libutmps and libskarnet (dependency of libutmps) are linked statically -
it's much more space efficient than linking them dynamically and I really
don't wanna add dynamic dependencies to busybox, the very core system
component, that currently depends on libc only. Also, author of these
libraries (skarnet) strongly recommends linking them statically.

libutmps is just a client library for utmps - one needs utmps-utmpd and
utmps-wtmpd daemons running to get the user accounting actually work.
However, it's completely optional, i.e. users don't have to install utmps
to use busybox as before.

See !25054