OK, this is a bit of a rewrite for the infrun code that
handles reverse stepping into/over shared library functions
and trampolines.  I believe we now correctly handle various
combinations of ...
   * reverse-step, reverse-next
   * trampolines, solib-dynsym-resolver
   * resolved and unresolved symbols
   * debuggable (user) and undebuggable (eg. libc) functions

This also includes the first testsuite test case for reverse
debugging.  If this one's accepted, I've got more queued up.

Hui, this patch will replace your patch 
http://sourceware.org/ml/gdb-patches/2009-03/msg00005.html,
so I'll appreciate your review.

Marc, this patch should fix all of your solib step/next bugs.

No regressions on linux-i686.