shared/path-lookup: rename user control dirs to "user.control"

shared/path-lookup: rename user control dirs to "user.control"

$ diff -u <(old/systemd-analyze --user unit-paths) <(new/systemd-analyze --user unit-paths)|colordiff
--- /proc/self/fd/14	2018-02-08 14:36:34.190046129 +0100
+++ /proc/self/fd/15	2018-02-08 14:36:34.190046129 +0100
@@ -1,5 +1,5 @@
-/home/zbyszek/.config/systemd/system.control
-/run/user/1000/systemd/system.control
+/home/zbyszek/.config/systemd/user.control
+/run/user/1000/systemd/user.control
 /run/user/1000/systemd/transient
 ...

Strictly speaking, online upgrades of user instances through daemon-reexec will
be broken. We can get away with this since
a) reexecs of the user instance are not commonly done, at least package upgrade
   scripts don't do this afawk.
b) cgroups aren't delegateable on cgroupsv1 there's little reason to use "systemctl
   set-property" for --user mode