Fix include and deprecated "register" keyword with Clang

I've been trying to compile my plugin with this new SDK, and there are two errors that stop it from compiling I could fix:
iclient.h: Remove userid.h include, it doesn't exist (gcc and clang)
mathlib.h: "register" is deprecated (clang)

There are a lot more errors related to telemetry, some files are missing, but I haven't looked into it much.