core: Non-zero values should set _armor, _textmode, and _online.

* src/gpgme.c (gpgme_set_armor, gpgme_set_textmode,
gpgme_set_offline): Ensure that non-zero values actually set the
appropriate internal bit.

The documentation for these functions says things like "disabled if
YES is zero, and enabled otherwise", where YES is an integer.
However, if you pass an even integer, it will be aliased back to 0
because the context member variables are declared as single bits.

This should make the implementation match the documentation.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>