Part of #7245: when KeyboardInterrupt is raised while defining commands, restore the old commands instead of producing a traceback.

Show the traceback line numbers as well as the current line numbers if an exception is being debugged.  Courtesy of pdb++ by Antonio Cuni.  Also document -> and >> markers for "list".

* added basic test for basic commands
* removed duplication of command docs, and moved them to their implementation
* unified and useful display of exceptions
* output messages and errors using overridable methods (also fixes #1503502)

Add Breakpoint.bpformat(), which returns the info usually printed by bpprint().  Necessary for major refactoring of pdb output handling.

#809887: improve pdb feedback for breakpoint-related actions.  Also add a functional test for these commands.

#6719: In pdb, do not stop somewhere in the encodings machinery if the source file to be debugged is in a non-builtin encoding.

been reassigned.

#1472251: remove addition of "\n" to code given to pdb.run[eval](), the bug in exec() that made this necessary has been fixed.  Also document that you can give code objects to run() and runeval(), and add some tests to test_pdb.

#8015: fix crash when entering an empty line for breakpoint commands.  Also restore environment properly when an exception occurs during the definition of commands.

#1437051: allow "continue"/"next"/etc. in .pdbrc, also add pdb -c option to give these commands.  This allows to run a script until an exception occurs.

#9230: allow Pdb.checkline() to be called without a current frame, for setting breakpoints before starting debugging.