Set GMT_keyword_update to false in gmt_begin

[maxrjones]

Description of proposed changes

This PR implements @PaulWessel's suggestion to use gmt_M_memset on GMT_keyword_updated to fix the issue summarized in this comment.

Fixes #

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[PaulWessel]

Did it actually work?

[maxrjones]

The minimal example that @seisman posted is fixed, but some other issues show up in the PyGMT tests. I will post over in the PyGMT issue for feedback.

[PaulWessel]

OK, since this helped and is surely the correct way to initialize memory (regardless if there are more problems found), please add me as reviewer and I will approve.