[lldb] [Process/FreeBSDRemote] Use RegSetKind consistently [NFC]

Use RegSetKind enum for register sets everything, rather than int.
Always spell it as 'RegSetKind', without unnecessary 'enum'.  Add
missing switch case.  While at it, use uint32_t for regnums
consistently.

Differential Revision: https://reviews.llvm.org/D93450