Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

The various types of cryptographic algorithm are fundamentally
different, and it was probably a mistake to try to handle them via a
single common type.

pubkey_algorithm is a placeholder type for now.

Use our own, more robust, ASN.1 parsing routines to extract the RSA
public key from a server certificate.  Remove the now-unused AXTLS
ASN.1 parser.

gcc 4.4 adds another few warnings, and also seems to complain if we
place %ebp in the clobber list for any inline asm.