emacs: Fix search refresh when on the last line of a search buffer.

We currently allow the cursor to be positioned on the "End of search
results" line after the last thread in a search buffer. When
refreshing on this line, there's no thread ID to be used as the
target.

Previously, a refresh from this case would result in a nil thread
target, but we were also using nil to indicate that the target thread
had been found. This caused the position to be lost during refresh,
(the cursor would move from the last line in the buffer to the first).

We fix this by using a magic string of "found" rather than nil to
clearly indicate whether the target thread has actually been found.