ICU: Mark which functions are threadsafe and which ones aren't

ICU: Mark which functions are threadsafe and which ones aren't

Mark the thread-safe functions with the \threadsafe doc marker. This
includes public API, which should be thread-safe anyway.

The thread-unsafe functions are marked "\nonreentrant" already. In
addition, I renamed the functions that must be called with locked
mutexes to Unlocked, following the convention in other libraries like
libdbus-1.

Change-Id: Ibd93d1266149767f546c8e82959b73c138008469
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>