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.
There is some duplication here. Basically the format of the two is the same, but only the wrapping is different.