install: allow specifiers in WantedBy/RequiredBy/Alias

This allows one templated unit to refer to another templated unit
at installation time.

Examples:

> grep WantedBy ~/.config/systemd/user/mpop@.timer
WantedBy=services@%i.target

> srv disable mpop@iit.timer
rm '/home/alxchk/.config/systemd/user/services@iit.target.wants/mpop@iit.timer'
> srv enable mpop@iit.timer
ln -s '/home/alxchk/.config/systemd/user/mpop@.timer' '/home/alxchk/.config/systemd/user/services@iit.target.wants/mpop@iit.timer'

Based-on-patch-by: Oleksii Shevchuk <alxchk@gmail.com>

src/core/unit-printf.c

@@ -250,12 +250,13 @@ char *unit_full_printf(Unit *u, const char *format) {
          * %r root cgroup path of this systemd instance (e.g. "/user/lennart/shared/systemd-4711")
          * %R parent of root cgroup path (e.g. "/usr/lennart/shared")
          * %t the runtime directory to place sockets in (e.g. "/run" or $XDG_RUNTIME_DIR)
+         * %U the UID of the configured user or running user
          * %u the username of the configured user or running user
          * %h the homedir of the configured user or running user
          * %s the shell of the configured user or running user
          * %m the machine ID of the running system
-         * %b the boot ID of the running system
          * %H the host name of the running system
+         * %b the boot ID of the running system
          */
 
         const Specifier table[] = {
@@ -282,7 +283,6 @@ char *unit_full_printf(Unit *u, const char *format) {
                 { 0, NULL, NULL }
         };
 
-        assert(u);
         assert(format);
 
         return specifier_printf(format, table, u);