Change tryopen to allow opening multiple files during one run.
Return number of errors at end.

Adapt test cases.

regress/open_cons_extrabytes.test

diff --git a/regress/open_cons_extrabytes.test b/regress/open_cons_extrabytes.test
index 8fdf2cc..1a74f67 100644
--- a/regress/open_cons_extrabytes.test
+++ b/regress/open_cons_extrabytes.test
@@ -4,3 +4,4 @@
 args -c testextrabytes.zip
 return 1
 stdout opening `testextrabytes.zip' returned error 21
+stderr 1 errors