tree-wide: stop using canonicalize_file_name(), use chase_symlinks() instead

Let's use chase_symlinks() everywhere, and stop using GNU
canonicalize_file_name() everywhere. For most cases this should not change
behaviour, however increase exposure of our function to get better tested. Most
importantly in a few cases (most notably nspawn) it can take the correct root
directory into account when chasing symlinks.

src/basic/mount-util.h

@@ -30,7 +30,7 @@
 #include "missing.h"
 
 int fd_is_mount_point(int fd, const char *filename, int flags);
-int path_is_mount_point(const char *path, int flags);
+int path_is_mount_point(const char *path, const char *root, int flags);
 
 int repeat_unmount(const char *path, int flags);