Switchlink header file cleanup

[ffoulkes]

These changes emerged from an effort to implement Bazel support for the Kernel Monitor.

• Some of them make the name of a header file better reflect its contents.
• Some of them remove dead code.
• Some of them reduce coupling and improve separation of concerns between between source and header files. (Reducing coupling is one of the things you can do to speed up the build.)
• One of them breaks a cyclical dependency between header files, which is something Bazel won't tolerate.
• And there are a few Boy Scout Rule edits, to leave the codebase better than I found it.

I know it looks like a lot, but I think you'll see a small number of patterns being repeated across the files. I rated it medium because of the number of files changed. In truth, it's probably a minor effort (15 minutes or less).

1. Renamed switchlink_handle.h to switchlink_handlers.h, to make it clear that this file defines interfaces to switchlink handlers, not switchlink_handle_t. (readability)
2. Renamed switchlink_link.h to switchlink_link_types.h, to make it clear that this file defines switchlink data types, and is not the header file for switchlink_link.c. (readability)
3. Moved declarations for switchlink_globals.c to a new switchlink_globals.h header file. (separation of concerns; cohesiveness)
4. Deleted unused SWITCHLINK_LOG_XXX and g_log_level definitions. (dead code)
5. Deleted unused definition of switchlink_get_nl_sock(). (dead code)
6. Deleted unused definition of switchlink_entry_type. (dead code)
7. Made switchlink_nhop_using_by a simple enum instead of a typedef. (reducing complexity and coupling.)

[nupuruttarwar]

LGTM

[5abeel]

LGTM