[RFA]: Fix for pending breakpoints in manually loaded/unloaded shlibs

[Andrew Cagney]

> To fix this, I have added an observer for a new event: solib_unloaded.  When update_solib_list discovers a shared library has been unloaded, it notifies all observers (initially this is just breakpoint.c).  Breakpoint.c sets up an observer to find all breakpoints in the removed shlib and mark them as non-inserted and bp_shlib_disabled.  This solves the problem.

The observer changes are ok (provided eli is ok with the doco description).

Andrew