Units should be added to the corresponding numbers

First dividing by interval before multiplying with 1000 decreases the
precision by +-(interval - 1) * 1000, as interval arithmetic always
applies the Gauß-function to the result.

This is not necessary and simply reordering the operations mitigates
this.

Within the components, snprintf() was unchecked and had inefficient
calls in some places.
We implement esnprintf() that does all the dirty laundry for us
and use it exclusively now.

implemented additional error condition for openbsd netstat in case the
interface could not be found in the interface list or if_data is not
readable.

implemented the netspeed functionality for openbsd.
furthermore the static keyword was removed of the interval variable in
config.def.h for usage as extern variable.

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.

1) Remove setlocale() (locales are harmful and any 'issues' shall
    be fixed in different ways that are expected).
 2) Disable buffering on stdout with setbuf() rather than flushing
    it each time.
 3) Make error messages more consistent.
 4) Add error checks where applicable.
 5) Make code a bit more readable where res is assigned.
 6) Use XFlush() rather than XSync() (we don't need to wait for the
    XServer to react, which could lead to long hangs on our side).

Posix guarantees that the resulting string is null-terminated, even if
we have an overflow. Instead of doing what has already been done,
properly warn when there has been an error or overflow, so the user can
do something about it.

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.