RSA: slight adjustment to CRYPT_RSA_PUBLIC_FORMAT_PKCS1_RAW format

CRYPT_RSA_PUBLIC_FORMAT_PKCS1_RAW and CRYPT_RSA_PUBLIC_FORMAT_PKCS1
produce two very similar looking keys but they are not the same.
As dissection OpenSSL's asn1parse would reveal CRYPT_RSA_PUBLIC_FORMAT_PKCS1
has the fact that it is an RSA key embedded within it whereas
CRYPT_RSA_PUBLIC_FORMAT_PKCS1_RAW does not. phpseclib now resolves
this ambiguity in the same way that OpenSSH's ssh-keygen does.

Despite this change CRYPT_RSA_PUBLIC_FORMAT_PKCS1_RAW is still incompatible
with OpenSSL's rsautl (CRYPT_RSA_PUBLIC_FORMAT_PKCS1 is compatible). I guess
this incompatibility isn't just due to the headers but is also due to the
overall structure of the format.