Emacs battery.el upower can report N/A, read as 0%

battery-format "%p" on the battery-upower backend can return "N/A" for a moment (for example, soon after boot or while D-Bus settles). string-to-number turns that into 0. Low-battery code then acts as if the battery is empty; in one setup it hibernated a laptop at 44% charge. On Linux, read /sys/class/power_supply/*/capacity and the online file of the type Mains supply instead. Also require two readings in a row at a level before any notification or hibernate.