Add warn() and die()

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.

util.h

@@ -3,5 +3,10 @@ extern char buf[1024];
 
 #define LEN(x) (sizeof (x) / sizeof *(x))
 
+extern char *argv0;
+
+void warn(const char *, ...);
+void die(const char *, ...);
+
 const char *bprintf(const char *fmt, ...);
 int pscanf(const char *path, const char *fmt, ...);