homed: allow overriding the root directory for home dirs via env var (i.e. use a different path than /home/)

This is a debugging feature. It's sometimes incredibly useful to be able
to run a second instance of homed that operates on another dir than
/home/.

Specifically, if you build homed from the source tree you can now run an
instance of it pretty reasonably directly from the build tree via:

  sudo SYSTEMD_HOME_DEBUG_SUFFIX=foo SYSTEMD_HOMEWORK_PATH=$(pwd)/build/systemd-homework SYSTEMD_HOME_ROOT=/home/foo ./build/systemd-homed

And then talk to it via

  sudo SYSTEMD_HOME_DEBUG_SUFFIX=foo homectl …

(you might need to tweak your dbus policy for this to work fully though)

src/basic/user-util.c

@@ -1085,3 +1085,14 @@ int is_this_me(const char *username) {
 
         return uid == getuid();
 }
+
+const char *get_home_root(void) {
+        const char *e;
+
+        /* For debug purposes allow overriding where we look for home dirs */
+        e = secure_getenv("SYSTEMD_HOME_ROOT");
+        if (e && path_is_absolute(e) && path_is_normalized(e))
+                return e;
+
+        return "/home";
+}

src/basic/user-util.h

@@ -112,6 +112,8 @@ bool is_nologin_shell(const char *shell);
 
 int is_this_me(const char *username);
 
+const char *get_home_root(void);
+
 /* A locked *and* invalid password for "struct spwd"'s .sp_pwdp and "struct passwd"'s .pw_passwd field */
 #define PASSWORD_LOCKED_AND_INVALID "!*"