psmisc: move killall to /usr/libexec and add ALTERNATIVES

Currently, this package can not be installed while using standard path
of busybox, because binary killall wants to be installed on the same
location as busybox.

Collision:
• /usr/bin/killall: busybox (new-file), psmisc (existing-file)

Many of these binaries, which provides alternatives were moved to
folder /usr/libexec like wget, sed, findutils, less.
So I moved killall to /usr/libexec and others leave in touch and added
ALTERNATIVES for it, because preinstall script is no longer necessary.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
(cherry picked from commit d8969e0f)