mountpoint-util: rename fd_is_mount_point() to is_mount_point_at()

fd_* functions in our codebase usually mean fd-specific operations,
while this one actually takes openat()-style params. Rename it
accordingly hence.

src/basic/mountpoint-util.h

@@ -49,7 +49,7 @@ static inline int path_get_mnt_id(const char *path, int *ret) {
         return path_get_mnt_id_at(AT_FDCWD, path, ret);
 }
 
-int fd_is_mount_point(int fd, const char *filename, int flags);
+int is_mount_point_at(int fd, const char *filename, int flags);
 int path_is_mount_point_full(const char *path, const char *root, int flags);
 static inline int path_is_mount_point(const char *path) {
         return path_is_mount_point_full(path, NULL, 0);