termproxy: install to usr/libexec directory

termproxy: install to usr/libexec directory

this is not a standard CLI tool that should be exposed to all users,
but an internal binary, so  move it to /usr/libexec/proxmox.
That location was chosen over a multiarch path
/usr/lib/<triplet>/proxmox to make resolving the path easier for the
caller, as the standard ways to get the multiarch triplet (e.g.,
"x86_64-linux-gnu") involve CC and are not easily/cheaply available in
perl – we would basically create an architecture specific package that
provides a path resolution with multiarch triplet resolved at "build"
time of that module; as we probably do not ever install termproxy
multiple times targeting different arch's on the same system, this is
just to much complexity.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>