refactor all stat functions in terms of fstatat

equivalent logic for fstat+O_PATH fallback and direct use of
stat/lstat syscalls where appropriate is kept, now in the fstatat
function. this change both improves functionality (now, fstatat forms
equivalent to fstat/lstat/stat will work even on kernels too old to
have the at functions) and localizes direct interfacing with the
kernel stat structure to one file.

src/stat/fstat.c

@@ -1,3 +1,4 @@
+#define _BSD_SOURCE
 #include <sys/stat.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -5,17 +6,7 @@
 
 int fstat(int fd, struct stat *st)
 {
-	int ret = __syscall(SYS_fstat, fd, st);
-	if (ret != -EBADF || __syscall(SYS_fcntl, fd, F_GETFD) < 0)
-		return __syscall_ret(ret);
-
-	char buf[15+3*sizeof(int)];
-	__procfdname(buf, fd);
-#ifdef SYS_stat
-	return syscall(SYS_stat, buf, st);
-#else
-	return syscall(SYS_fstatat, AT_FDCWD, buf, st, 0);
-#endif
+	return fstatat(fd, "", st, AT_EMPTY_PATH);
 }
 
 weak_alias(fstat, fstat64);