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 (EDIT: and I really don’t wanna add dynamic dependencies to busybox, the very core system component, that currently depends on libc only).

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.

/cc @dbradley

EDIT: I’ve consulted it with @skarnet and he said that this MR is correct and shouldn’t break anything.