Add system-info Linux systemd user unit

Update the `system-info` directory with the addition of a `systemd`
service file and an new Linux script to display current CPU temperature
and core/thread utilization. Update `.conduit.yaml` with addition
`apt` dependencies `sysstat` and `jq`. Update the install and remove
scripts to setup and tear down the `systemd` user unit. Lastly, decrease
the `tmux` status line update interval from 5 to 2 seconds.

system-info/system-info-Linux.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+cache_dir=~/.cache/tmux
+cache_file=$cache_dir/system-info
+
+# Make sure the output directory exists.
+if [ ! -d $cache_dir ]; then
+  mkdir -p $cache_dir
+fi
+
+# Cleanup cache file when interrupted.
+trap '[ -f $cache_file ] && rm $cache_file; exit' INT
+trap '[ -f $cache_file ] && rm $cache_file; exit' TERM
+
+while true; do
+  # Parse the current CPU load on all cores/threads.
+  cpu_load=`mpstat -P ALL -n 1 -u 1 -o JSON | \
+    jq '.sysstat.hosts[0].statistics[0]["cpu-load"][1:]|.[].idle' | \
+    awk '$idle ~ /[-.0-9]*/ { printf "%s", substr("█▇▆▅▄▃▂▁ ", int($idle / 11), 1) }'`
+  # Parse the current CPU package temperature.
+  cpu_temp=`sensors coretemp-isa-0000 | awk 'NR == 3 { print substr($4, 2) }'`
+  # Write to the cache file.
+  echo "$cpu_temp $cpu_load" > $cache_file
+done