Split into multiple files

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

hostname.c

@@ -0,0 +1,15 @@
+#include <err.h>
+#include <unistd.h>
+
+#include "util.h"
+
+const char *
+hostname(void)
+{
+	if (gethostname(buf, sizeof(buf)) == -1) {
+		warn("hostname");
+		return NULL;
+	}
+
+	return buf;
+}