library/dhm.c: accept (and ignore) optional privateValueLength for
PKCS#3 DH parameters.

PKCS#3 defines the ASN.1 encoding of a DH parameter set like this:

----------------
DHParameter ::= SEQUENCE {
  prime INTEGER, -- p
  base INTEGER, -- g
  privateValueLength INTEGER OPTIONAL }

The fields of type DHParameter have the following meanings:

     o    prime is the prime p.

     o    base is the base g.

     o    privateValueLength is the optional private-value
          length l.
----------------

See: ftp://ftp.rsasecurity.com/pub/pkcs/ascii/pkcs-3.asc

This optional parameter was added in PKCS#3 version 1.4, released
November 1, 1993.

dhm.c currently doesn't cope well with PKCS#3 files that have this
optional final parameter included. i see errors like:

------------
dhm_parse_dhmfile returned -0x33E6

Last error was: -0x33E6 - DHM - The ASN.1 data is not formatted correctly : ASN1 - Actual length differs from expected lengt
------------

You can generate PKCS#3 files with this final parameter with recent
versions of certtool from GnuTLS:

 certtool --generate-dh-params > dh.pem