gdb/python/dap: prefix internal attributes with underscore

I'm currently reading the DAP code, and I think this would help.  This
is pretty much standard Python style, we do it as some places but not
others.  I think it helps readability, by saying that this attribute
isn't mean to be accessed outside the class.

A similar pass could be done for internal methods, I haven't done that.

Change-Id: I8e8789b39adafe62d14404d19f7fc75e2a364e01
Approved-By: Tom Tromey <tom@tromey.com>