fix init script
pass battery information as environment variables

Set BATTERY, BATTERY_PERCENT, ..., FALLBACK_BATTERY, ... instead of passing
args to scripts.
That way it's easier to extend (and scripts are more readable).
(It also allows to pass arbitrary args to scripts, in future.)

* Added some vars:
  - remaining running time (battery)
  - status (fallback battery)
    (for battery, it'd be either "unknown" or "discharging")
  - remaining charging time (fallback battery)

* --no-args, -0 option has been removed (but not script_type)

* use gcc __attribute__ if available (noreturn, warn_unused_result)
event-scripts/: use env vars
update README
Makefile: dist target, improved script generation

* perform basic syntax checking ($shell -n <file>) on generated files
* dist target for creating source tarballs (and checksums)
* genclean / distclean targets
instagit mode!

Simply download && execute the instagitlet script to get batwatch up and
running.
"instagitlet.sh --dry-run" can be used to show what would be done.
See instagitlet.sh -h for usage details.