Fix security issues in pre-install script

- Having $HOME in /tmp means everyone can create e.g. a .bashrc in your home dir
- "chown -R" on the logdir is problematic when an attacker takes over alppaca
  and places hardlinks to files that alppaca should not be able to access in
  that directory. The next time the pre-install script runs, it would chown
  the hardlinked file to alppaca.
- Having /etc/alppaca writable for alppaca itself means that an attacker that
  takes over alppaca can re-configure alppaca to run as root and then take
  over the privileged alppaca process.

pre-install.sh

@@ -5,20 +5,17 @@ confdir=/etc/alppaca
 
 # add service user and group
 groupadd $user 2> /dev/null || :
-useradd -c "Alppaca - a local prefetch proxy for amazon credentials" -s /sbin/nologin -r -d /tmp -g $user $user 2> /dev/null || :
+useradd -c "Alppaca - a local prefetch proxy for amazon credentials" -s /sbin/nologin -r -d / -g $user $user 2> /dev/null || :
 
 # Add log directory
 if [ ! -d $logdir ]; then
   mkdir -p $logdir
 fi
 
 # change ownership of directory to $user
-chown -R $user: $logdir
+chown $user: $logdir
 
 # Add the conf directory
 if [ ! -d $confdir ]; then
   mkdir -p $confdir
 fi
-
-# change ownership of directory to $user
-chown -R $user: $confdir