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

user.c

@@ -0,0 +1,31 @@
+#include <err.h>
+#include <pwd.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "util.h"
+
+const char *
+gid(void)
+{
+	return bprintf("%d", getgid());
+}
+
+const char *
+username(void)
+{
+	struct passwd *pw = getpwuid(geteuid());
+
+	if (pw == NULL) {
+		warn("Failed to get username");
+		return NULL;
+	}
+
+	return bprintf("%s", pw->pw_name);
+}
+
+const char *
+uid(void)
+{
+	return bprintf("%d", geteuid());
+}