Use new default-user-shell option instead of hard-coding bash in nspawn and user-record

Defaults to /bin/bash, no changes in the default configuration

The fallback shell for non-root users is as-specified,
and the interactive shell for nspawn sessions is started as
  exec(default-user-shell, "-" + basename(default-user-shell), ...)
before falling through to bash and sh

src/shared/user-record.c

@@ -1747,7 +1747,7 @@ const char *user_record_shell(UserRecord *h) {
                 return "/bin/sh";
 
         if (user_record_disposition(h) == USER_REGULAR)
-                return "/bin/bash";
+                return DEFAULT_USER_SHELL;
 
         return NOLOGIN;
 }