wall: always use utmp as fallback

wall: always use utmp as fallback

Wall(1) can be compiled with systemd support to read the names of ttys
from the systemd session list. However, this may not work on all systems.
In case of failure, the best option is to use the traditional
utmp method as a fallback.

This commit uses strv (string vector) to collect tty names from both
sources (systemd and utmp) and then sends a message to all ttys.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=2283049
Signed-off-by: Karel Zak <kzak@redhat.com>