nspawn: rename pin_fully_visible_fs() → pin_fully_visible_api_fs()

This function pins the *API* FS, i.e. /proc/ + /sys/, not just any fs.
Hence clarify this in the name.

(At least we call these two fs "API (V)FS" in our codebase, hence
continue to do so here)

src/nspawn/nspawn-mount.h

@@ -73,5 +73,6 @@ int pivot_root_parse(char **pivot_root_new, char **pivot_root_old, const char *s
 int setup_pivot_root(const char *directory, const char *pivot_root_new, const char *pivot_root_old);
 
 int tmpfs_patch_options(const char *options,uid_t uid_shift, const char *selinux_apifs_context, char **ret);
-int pin_fully_visible_fs(void);
-int wipe_fully_visible_fs(int mntns_fd);
+
+int pin_fully_visible_api_fs(void);
+int wipe_fully_visible_api_fs(int mntns_fd);