Add longoptions support for gmtvector.

[rbdavis]

Description of proposed changes

Python differences:

No python module.

Julia differences:

Replaced -A single_vec with primary_vector. The man page refers
to a single vector in its description of both the -A and -S options
('single primary' in the former case, and 'single secondary' in the
latter), hence it's misleading to refer only to -A as 'single_vec'.

Fixes #8068

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]

Seems fine to me.