rsa1: rsa_loadpriv_core: Introduce rsapriv_load_ctx

rsa1: rsa_loadpriv_core: Introduce rsapriv_load_ctx

Make the LD* macros call a new function load_error, and `goto
error_out'.

We must add a call to rsapriv_dispose on the error_out path so that
rsapriv_load_core now always frees everything at every LD* point - so
now this is OK from a memory management pov.

load_error calls a hook function verror.  This is going to let us
parameterise the way rsapriv_load_core works.

No overall functional change yet.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>