Fix parsing of serial# in req

Reported by Jakub Wilk.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2556)

apps/req.c

@@ -121,7 +121,7 @@ const OPTIONS req_options[] = {
     {"multivalue-rdn", OPT_MULTIVALUE_RDN, '-',
      "Enable support for multivalued RDNs"},
     {"days", OPT_DAYS, 'p', "Number of days cert is valid for"},
-    {"set_serial", OPT_SET_SERIAL, 'p', "Serial number to use"},
+    {"set_serial", OPT_SET_SERIAL, 's', "Serial number to use"},
     {"extensions", OPT_EXTENSIONS, 's',
      "Cert extension section (override value in config file)"},
     {"reqexts", OPT_REQEXTS, 's',

doc/man1/req.pod

@@ -242,7 +242,6 @@ days to certify the certificate for. The default is 30 days.
 
 serial number to use when outputting a self signed certificate. This
 may be specified as a decimal value or a hex value if preceded by B<0x>.
-It is possible to use negative serial numbers but this is not recommended.
 
 =item B<-extensions section>