* Implement posix-sema.c

* Allow to use GTK's main loop instead of the select stuff in
  wait.c

* add locking to the key cache?

* Should --delete silently delete secret keys or is there a need for
  another flag or a callback?

* GpgmeKey misses GPGME_ATTR_EXPIRE attribute

* Add ATTR to return the number of subkeys or uids.

* Return GPGME_Canceled when appropriate

* Factor out common code in _op_*_start functions.

* Move code common to all engines up from gpg to engine.

* Tests
** t-data
*** Test gpgme_data_release_and_get_mem.
*** Test gpgme_data_rewind for invalid types.
*** Test gpgme_data_read's readable feature.

* Changes breaking the ABI
** GpgmeKey's main userID is not index == 0.

Bugs reported by Stephane Corthesy:
> - When returning a GpgmeKey GPGME_ATTR_COMMENT attribute, characters  
> like ":" are not un-escaped, they are returned as \x3a

> BTW, here's another bug: it it not possible to retrieve fingerprints  
> for subkeys

> In GpgmeRecipients, would it be possible to provide a function which  
> would return the validity assigned to a name contained in the  
> GpgmeRecipients instance?

> - There is an inconsistent behaviour: if we pass three times an  
> invalid (but non empty) passphrase, return code is GPGME_No_Data, but  
> if we pass three times an empty (and invalid) passphrase, we get  
> GPGME_No_Passphrase.

> passphrase callback. If I use the same GpgmeContext as the one which  
> is currently asking for a passphrase, my app crashes: the r_hd in
> the  
> callback has become invalid; if I use a brand new one, the callback  
> is called recursively, when I ask to enumerate keys.