Fix gdb-mi.el bug#9853, bug#9858, and bug#9878.

Fix gdb-mi.el bug#9853, bug#9858, and bug#9878.

* lisp/progmodes/gdb-mi.el (gdb-version): Remove defvar.
(gdb-supports-non-stop): New defvar, replacing `gdb-version'.
(gdb-gud-context-command, gdb-non-stop-handler)
(gdb-current-context-command, gdb-stopped): Use it.
(gdb-init-1): Enable pretty printing here.
(gdb-non-stop-handler): Don't enable pretty-printing here.  Check
to see if the target supports non-stop mode; if not, turn off
non-stop mode.  Use the following.
(gdb-check-target-async): New defun.
(gud-watch, gdb-stopped): Fix whitespace.
(gdb-get-source-file): Don't try to display the source file if
`gdb-main-file' is nil.