This adds an event that is emitted just before GDB presents a prompt
to the user. This provides Python code a way to react to whatever
changes might have been made by the previous command. For example, in
my GUI I use this to track changes to the selected frame and reflect
them in the UI.
Built and regtested on x86-64 Fedora 23.
gdb/ChangeLog
2017-02-14 Tom Tromey <tom@tromey.com>