Add gpgme_set_global_flag to help debugging

Add gpgme_set_global_flag to help debugging

* src/gpgme.c (gpgme_set_global_flag): New.
* src/gpgme.h.in (gpgme_set_global_flag): New.
* src/gpgme.def, src/libgpgme.vers: Add new public function.
* src/debug.c (envvar_override): New.:
(_gpgme_debug_set_debug_envvar): New.
(debug_init): Take ENVVAR_OVERRIDE in account.
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On Android envvars can't be used, thus we need another way to enable
GPGME debugging.  The new function allows this and may be used in the
future to implement similar things.