daemon: Make 'profiles/per-user' non-world-writable.

Fixes <https://bugs.gnu.org/37744>.
Reported at <https://www.openwall.com/lists/oss-security/2019/10/09/4>.

Based on Nix commit 5a303093dcae1e5ce9212616ef18f2ca51020b0d
by Eelco Dolstra <edolstra@gmail.com>.

* nix/libstore/local-store.cc (LocalStore::LocalStore): Set 'perUserDir'
to #o755 instead of #o1777.
(LocalStore::createUser): New function.
* nix/libstore/local-store.hh (LocalStore): Add it.
* nix/libstore/store-api.hh (StoreAPI): Add it.
* nix/nix-daemon/nix-daemon.cc (performOp): In 'wopSetOptions', add
condition to handle "user-name" property and honor it.
(processConnection): Add 'userId' parameter.  Call 'store->createUser'
when userId is not -1.
* guix/profiles.scm (ensure-profile-directory): Note that this is now
handled by the daemon.
* guix/store.scm (current-user-name): New procedure.
(set-build-options): Add #:user-name parameter and pass it to the daemon.
* tests/guix-daemon.sh: Test the creation of 'profiles/per-user' when
listening on a TCP socket.
* tests/store.scm ("profiles/per-user exists and is not writable")
("profiles/per-user/$USER exists"): New tests.

nix/libstore/store-api.hh

@@ -289,6 +289,10 @@ public:
     /* Check the integrity of the Nix store.  Returns true if errors
        remain. */
     virtual bool verifyStore(bool checkContents, bool repair) = 0;
+
+    /* Create a profile for the given user.  This is done by the daemon
+       because the 'profiles/per-user' directory is not writable by users.  */
+    virtual void createUser(const std::string & userName, uid_t userId) = 0;
 };