On the svn-auth-x590 branch, constify pointers used within cert parser code.
Also fix some quirks in use of unsigned types.

* subversion/libsvn_subr/x509.h
  (x509_buf): The 'p' pointer in here needs 'const'.
  (svn_x509_parse_cert): Remove 'unsigned' from the char * in this API.
   'unsigned' types are used because an ASN1 constant has the high bit set and
   is used in logical comparisons. We'll hide this quirk inside the parser.

* subversion/libsvn_subr/x509parse.c
  (asn1_get_len, asn1_get_tag, asn1_get_int, x509_get_version,
   x509_get_serial, x509_get_alg, x509_get_name, x509_get_dates,
   x509_get_sig, x509_get_uid): Add 'const'.
  (svn_x509_parse_cert): Remove 'unsigned' from input argument.
   Internally cast the pointer to 'unsigned' instead.

Suggested by: breser


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1603879 13f79535-47bb-0310-9956-ffa450edef68