- Remove <errno.h> because related functions are in util.c now
 - Remove corresponding <string.h> if not used otherwise

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.

Make use of strerror(errno) and format all errors equally:
function ['parameters']: error message

Replace warn() and warnx() with fprintf() and add <stdio.h> where
necessary.

This brings us a lot more tidiness.

For multiple reasons the program is now split:
- Make future porting to OpenBSD easier
- Assign header includes to individiual functions
- Make future program extensions easier
- Recompile only changed parts