Add public function gpgme_set_pinentry_mode.

Add public function gpgme_set_pinentry_mode.

* src/gpgme.c (gpgme_set_pinentry_mode): New.
* src/gpgme.h.in (gpgme_pinentry_t): New.
(gpgme_set_pinentry_mode): New.
* src/context.h (struct gpgme_context): Add field pinentry_mode.
* src/engine-backend.h (struct engine_ops): Add field
set_pinentry_mode.
* src/engine-gpg.c (struct engine_gpg): Add field pinentry_mode.
(build_argv): Implement pinentry_mode.
(gpg_set_pinentry_mode): New.
(_gpgme_engine_ops_gpg): Register gpg_set_pinentry_mode.

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Note that this new fucntion may only be used with gpg 2.1.