Implement explicit locations for Python breakpoints.

This introduces several new keywords to the bppy_init constructor.
The spec parameter is now optional but mutually exclusive to the
explicit keywords source, label, function and line.

gdb/ChangeLog

2017-12-07  Phil Muldoon  <pmuldoon@redhat.com>

       * python/py-breakpoint.c (bppy_init): Use string_to_event_location
       over basic location code. Implement explicit location keywords.
       (bppy_init_validate_args): New function.
       * NEWS: Document Python explicit breakpoint locations.

doc/ChangeLog

2017-12-07  Phil Muldoon  <pmuldoon@redhat.com>

       * python.texi (Breakpoints In Python): Add text relating
       to allowed explicit locations and keywords in gdb.Breakpoints.

testsuite/ChangeLog

2017-12-07  Phil Muldoon  <pmuldoon@redhat.com>

       * gdb.python/py-breakpoint.exp (test_bkpt_explicit_loc): Add new
       tests for explicit locations.