This patch makes the MI -thread-list-ids report the current thread.
It is advantageous for a frontend like Eclipse, that does not report
any details about a thread whatsoever except for the id, and whether
a thread is selected. If -thread-list-ids reports current thread, then
Eclipse can use -thread-list-ids as opposed to 'info thread', and be
a little bit faster.

The change in thread.c actually touches MI-only function, so I assume
I'm free to make this change. Eli, does the doc change look good?

- Volodya