Improve error diagnostics

* src/names.c (file_list_name): New static function.
(add_file_id): If a filelist is being read twice, print the names
of the files that caused it.
* tests/T-rec.at: New test case.
* tests/Makefile.am: Add new file.
* tests/testsuite.at: Include new file.

tests/testsuite.at

@@ -199,6 +199,7 @@ m4_include([opcomp06.at])
 AT_BANNER([The -T option])
 m4_include([T-mult.at])
 m4_include([T-nest.at])
+m4_include([T-rec.at])
 m4_include([T-cd.at])
 m4_include([T-empty.at])
 m4_include([T-null.at])