opendir() returns a directory stream, not a file descriptor

Co-authored-by: drkhsh <me@drkhsh.at>
Signed-off-by: drkhsh <me@drkhsh.at>

Fixes coding style. Formatting commits suck, incoherent coding style
sucks more.
https://suckless.org/coding_style/

Except stated otherwise in comment.

Fixes coding style. Formatting commits suck, incoherent coding style
sucks more.
https://suckless.org/coding_style/

this gives the compiler a chance to check weather the prototype and
definiton matches or not, which would catch issues like 3c47701.

 - 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.

- Sort Alphabetically
- Same indentation for preprocessor clauses

- 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