POSIX dyld: disable the entry breakpoint as soon as it gets hit.

Also mark it as one-shot.  We're not tracking it at this point anyway.
The one-shot is not enough to have the breakpoint cleared by the second
hit of the entry breakpoint we're seeing.

This is merely a clean-up change to make the real issue more apparent
for this bug:
https://github.com/tfiala/lldb/issues/71