Make -CAserial a type 's' option

The file name given to -CAserial might not exist yet.  The
-CAcreateserial option decides if this is ok or not.

Previous to this change, -CAserial was a type '<' option, and in that
case, the existence of the file given as argument is tested quite
early, and is a failure if it doesn't.  With the type 's' option, the
argument is just a string that the application can do whatever it
wants with.

Reviewed-by: Rich Salz <rsalz@openssl.org>

apps/x509.c

diff --git a/apps/x509.c b/apps/x509.c
index fe9b109..5938b43 100644
--- a/apps/x509.c
+++ b/apps/x509.c
@@ -175,7 +175,7 @@
      "The CA key, must be PEM format; if not in CAfile"},
     {"CAcreateserial", OPT_CACREATESERIAL, '-',
      "Create serial number file if it does not exist"},
-    {"CAserial", OPT_CASERIAL, '<', "Serial file"},
+    {"CAserial", OPT_CASERIAL, 's', "Serial file"},
     {"set_serial", OPT_SET_SERIAL, 's', "Serial number to use"},
     {"text", OPT_TEXT, '-', "Print the certificate in text form"},
     {"C", OPT_C, '-', "Print out C code forms"},