feature: more system variables

[NotAShelf]

In v1 the Watt configuration manager has been revamped to be a "rule evaluator" daemon. This system does not allow users to fetch data on their own, so it's good to expose some variables in the DSL.

Things I want to add, in no particular order:

Current Software Frequency Limits

• $cpu-scaling-maximum - current software max frequency (from scaling_max_freq)
• $cpu-scaling-minimum - current software min frequency (from scaling_min_freq) obsolete but maybe good for consistency

CPU Core Count

• $cpu-count - total CPU cores (probably not useful on single-node configs, but might allow sharing configs)

Load Averages

• $load-average-1m - 1-minute load average
• $load-average-5m - 5-minute load average
• $load-average-15m - 15-minute load average

Kind of redundant, possibly.

Battery Health

• %battery-health - battery health percentage (eh)
• $battery-cycles - battery charge cycles (eh)

Laptop State

• ?lid-closed - boolean for laptop lid state, can be used to aggressively slow down a system while the lid is down to save power in your bag or whatever

Time-of-day

• $hour-of-day - current hour (0-23), only useful if we want to allow scripting and would be obsoleted by a DBus interface