#56 unified naming approach for functions setting callbacks

[Martin Peylo]

There are the following functions setting callbacks

CMP_CTX_set_msg_transfer()
CMP_CTX_set_certConf_callback()
CMP_CTX_set_error_callback()
CMP_CTX_set_debug_callback()

At least one of them ;-) needs to be renamed to match the logic of the others.

OpenSSL is using mostly "cb" when getting/setting the callback functions, rarely expanded to "cb_func", so maybe we should also shorten that?

E.g.:
EVP_PKEY_set_cb()
X509_STORE_set_verify_cb_func()
X509_STORE_get_verify_cb()
X509_STORE_CTX_get_verify_cb()
X509_STORE_CTX_set_verify_cb()