Stdio.Fd: Map _errno.

Make it possible to set the errno for a Stdio.File from Pike code.

Also added some minimal documentation for the other mapped fields
in Stdio.Fd, as well as moving the documentation for the low-level
functions from Stdio.File to Stdio.Fd.
Added validation method and _sprintf.
Verify DH parameters.
Tidy up the signature verification a little.
Set keyAgreement flag on ECDSA certificates.
Document Program.all_inherits and give an example
Corrected comment.
Make make_key_usage_flags internal for now.
Removed one level of indirection and inherit SSL.Port directly into SSLPort.
All deprecated last.