[!99] replace hostname with uname -n & hostnamectl

The former belongs to package `inetutils`, which has some security
implications, so let's use `uname` from `coreutils` to get the hostname.

For *setting* the hostname things are more limited. Let's use
`hostnamectl` from `systemd` if available. Fall back to `hostname`,
then `sysctl`.

client/scripts/linux

@@ -113,7 +113,7 @@ set_hostname() {
     local current_hostname
 
     if [ -n "$new_host_name" ]; then
-        current_hostname=$(hostname)
+        current_hostname=$(uname -n)
 
         # current host name is empty, '(none)' or 'localhost' or differs from new one from DHCP
         if [ -z "$current_hostname" ] ||
@@ -121,7 +121,13 @@ set_hostname() {
            [ "$current_hostname" = 'localhost' ] ||
            [ "$current_hostname" = "$old_host_name" ]; then
            if [ "$new_host_name" != "$old_host_name" ]; then
-               hostname "$new_host_name"
+               if command -v hostnamectl >/dev/null; then
+                   hostnamectl set-hostname --transient "$new_host_name"
+               elif command -v hostname >/dev/null; then
+                   hostname "$new_host_name"
+               else
+                   sysctl -w kernel/hostname="$new_host_name"
+               fi
            fi
         fi
     fi