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/stat.c

@@ -4,11 +4,7 @@
 
 int stat(const char *restrict path, struct stat *restrict buf)
 {
-#ifdef SYS_stat
-	return syscall(SYS_stat, path, buf);
-#else
-	return syscall(SYS_fstatat, AT_FDCWD, path, buf, 0);
-#endif
+	return fstatat(AT_FDCWD, path, buf, 0);
 }
 
 weak_alias(stat, stat64);