Also uses size_t as the length indicator type, as is common with buffers.

Currently, this is only ever queried, so adding a function to check if the
server is enabled is more sensible.

If directly modifying this externally is ever desirable, it should be done
by adding a function to the interface, rather than exposing implementation
details directly.

This fixes the comments left on the PR (whitespace, SO_REUSEADDR,
comment changes).

Previously, the padding wasn't correctly accounted for which caused the gdbstub to read and write everything after R15 (starting with the dummy FPA registers) incorrectly, which caused CPSR to not be handled correctly. Everything appears to be working as expected with this change.