GPGME: const-ifying cbs argument for gpgme_data_new_from_cbs()

[Daniel Kahn Gillmor]

Hey all--

i'm playing with gpgme using callbacks, and i noticed that
gpgme_data_new_from_cbs takes its cbs argument (the struct of callback
function pointers) as non-const.  is there any reason that it plans to
modify this object or is the lack of constness just an oversight?

What is the expected lifespan of the cbs callback table?  Can it be
freed (or fall off the stack) before the gpgme_data_t is released?
Should these expected constraints be documented?  I'm up for improving
the documentation if someone wants to explain the expectations here.

Regards,

      --dkg