Initialize plugins after handling initial command line flags

This is technically a breaking change, since attempting to set plugin
files after the first non-flag argument will now throw an error. This
is acceptable given the relative lack of stability in a plugin
interface and the need to tie the knot somewhere once plugins can
actually define new subcommands.

src/build-remote/build-remote.cc

@@ -53,6 +53,9 @@ static int main_build_remote(int argc, char * * argv)
         unsetenv("DISPLAY");
         unsetenv("SSH_ASKPASS");
 
+        /* If we ever use the common args framework, make sure to
+           remove initPlugins below and initialize settings first.
+        */
         if (argc != 2)
             throw UsageError("called without required arguments");