Add some states not handled before. XXX: are both states in the | cases
possible?

branches: 1.7.6; 1.7.12;
Add cases for new {high,maximum}-capacity events

Long-overdue update. Need to handle both {warning,critical}-capacity
events.

Also log the 'low-power' event via logger -p warning to have records
of what happened.

Fix the problems described in:
http://mail-index.netbsd.org/current-users/2007/07/16/0012.html

- Introduce sme_class into the sysmon_envsys struct to specify a
class; currently there are two classes: SME_CLASS_ACADAPTER and
SME_CLASS_BATTERY.

- Add a new envsys event: PENVSYS_EVENT_LOW_POWER that is reached when
all SME_CLASS_BATTERY devices are in CRITICAL/LOW state and there's not
any SME_CLASS_ACADAPTER connected.

- Add the 'low-power' event into the sensor_battery script that will
shutdown the system gracefully via 'shutdown -p'. If powerd(8) is
not running, cpu_reboot(9) with RB_POWERDOWN is used.

- Make acpiacad(4) a SME_CLASS_ACADAPTER device and acpibat(4) a
SME_CLASS_BATTERY device.

Update the documentation accordingly to these changes.

Only three events are reported now:

normal
state-changed
user-capacity

But there's a fourth argument passed to the script that will report
current state description.

branches: 1.2.4;
Do not shutdown the system when a critical event is received, this
doesn't work when there are two or more batteries in acpibat(4).

Imported envsys 2, a brief description of the new features:
(Part 4: documentation and configuration files)

* Support for detachable sensors.
* Cleaned up the API for simplicity and efficiency.
* Ability to send capacity/critical/warning events to powerd(8).
* Adapted all the code to the new locking order.
* Compatibility with the old envsys API: the ENVSYS_GTREINFO
and ENVSYS_GTREDATA ioctl(2)s are supported.
* Added support for a 'dictionary based communication channel' between
sysmon_power(9) and powerd(8), that means there is no 32 bytes event
size restriction anymore.
* Binary compatibility with old envstat(8) and powerd(8) via COMPAT_40.
* All drivers with the n^2 gtredata bug were fixed, PR kern/36226.

Tested by:

blymn: smsc(4).
bouyer: ipmi(4), mfi(4).
kefren: ug(4).
njoly: viaenv(4), adt7463.c.
riz: owtemp(4).
xtraeme: acpiacad(4), acpibat(4), acpitz(4), aiboost(4), it(4), lm(4).