meson: don't search PATH for telinit (#7770)

On a typical system running systemd, the telinit in PATH is very likely to be a symlink
to systemctl. Setting TELINIT to this may result in an infinite recursion if telinit is called
and sd_booted() == 0. This may commonly occur in a chroot environment.

Bug: https://bugs.gentoo.org/642724

[zj:
The path was originally hardcoded as "/lib/upstart/telinit", but was made configurable without
changing the default in 4ad61fd1. Then the default was
changed to `/lib/sysvinit/telinit` in abaaabf4. Then it
started being  autodetected when meson support was added in 
5c23128d. This patch restores the behaviour that was 
implemented in configure.ac at the time of its removal.]