Add "local_generator"

This generator can be used by .spec file, which ships their own
generators:

~~~
Source1: generator.req
%global __local_generator_requires bash %{SOURCE1}
~~~

Resolves rpm-software-management#782

build/rpmfc.c

@@ -1184,10 +1184,11 @@ static int initAttrs(rpmfc fc)
 
     /* Discover known attributes from pathnames + initialize them */
     if (rpmGlob(attrPath, NULL, &files) == 0) {
-	nattrs = argvCount(files);
+	nattrs = argvCount(files) + 1;
 	fc->atypes = xcalloc(nattrs + 1, sizeof(*fc->atypes));
-	for (int i = 0; i < nattrs; i++) {
-	    char *bn = basename(files[i]);
+	fc->atypes[0] = rpmfcAttrNew("local_generator");
+	for (int i = 1; i < nattrs; i++) {
+	    char *bn = basename(files[i - 1]);
 	    bn[strlen(bn)-strlen(".attr")] = '\0';
 	    fc->atypes[i] = rpmfcAttrNew(bn);
 	}