Additionally unify the format of battery_state and uptime

when an AC is connected apm_info shows a non-valid value for remaining
minutes. it was decided that in that case the function should return an
empty string.

Tabs for indentation, spaces for alignment

Implementation of a battery_remaining function which returns the
remaining battery time in HH:MM format. Linux function still needs
implementation.
Move common code to load_apm_power_info

Given slstatus is a tool that runs in the background, most likely run
from .xinitrc, it's important to prepend the name of the tool to error
messages so it becomes clear where the error is coming from.

To make this much more consistent, this commit adds warn() and die()
utility functions consistent with other suckless projects and adapts all
calls to fprintf(stderr, *) to the warn() and die() functions, greatly
increasing the readability of the code.

Use variables named unused instead which is simpler

<sys/select.h> is needed before <net80211/ieee80211_ioctl.h> on OpenBSD
to define NBBY and shut up compiler warnings.

Only initialize variables at the beginning of a block

int for normal indexes
size_t for usage with the LEN macro

Use block for single statement ifs
When functions return -1 for error test against 0 not -1
Newlines

display ESSID and signal strength percentage

The swapctl(2) function fills the swapent struct with 512KB blocks.
As we want to display in GB, i just modified the calculation for this to
get the expected output.

sys/sensors.h has two structs struct timeval: sensor and ksensor:

	struct sensor {
		...
		struct timeval tv;		/* sensor value last change time */
		...
	};

- Sort Alphabetically
- Same indentation for preprocessor clauses

OpenBSD's entropy design is superior to Linux.

- Use block for single statement ifs
- Keep lines to reasonable length (current debate as to reasonable)
- When functions return -1 for error test against 0 not -1
- Do not indent cases another level
- Do not test against NULL and 0 explicitly
- Use tabs for indentation, use spaces for alignment

The third value from load_avg (idle) gives us almost the same
information as cpu_iowait. Plus OpenBSD does not offer an iowait value
as Linux and thus the corresponding function would not be portable.

In OpenBSD the CPU usage in percent is now computed using KERN_CPTIME
sysctl.

It does not respect charging thresholds and it is not trivial to
implement the indicator in OpenBSD.