When --show-below-main=no, search main from the outermost stackframe

When --show-below-main=no, search main from the outermost stackframe

* main is more likely to be an outermost frame rather than an innermost
  frame. So, searching from the outermost frame will more quickly find it.

* Also, in case the stacktrace contains twice the main functionn, this
  ensures we only removes the functions below the outermost main.
  Having 2 mains in a stacktrace does not happen normally.
  However, this prepares for some future commit that improves
  the outer/inner setup: the outer will append the inner guest stack trace.
  The inner stack trace sometimes already contains main.
  Searching from outermost frame main allows to keep the interesting
  part of the stacktrace.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16135