[RFC v5 0/8] enable target-async by default

[Tom Tromey <tromey at redhat dot com>]

This is version 5 of the patch series to enable target-async by
default.  Version 4 is here:

    https://sourceware.org/ml/gdb-patches/2013-10/msg00691.html

Compared to version 4 this:

* Has been rebased onto master, with this regression fix applied:
    https://sourceware.org/ml/gdb-patches/2014-03/msg00068.html

* Has had all review comments addressed, as far as I can tell; in
  particular:

  * Patch #5 now includes a standalone test case.

  * Patch #7 has more comments and a simplified condition in the
    prompt printing code.

  * Patch #7 also now splits the back-end and MI notion of async;
    putting the back-end notion on a new "maint" command so that
    maintainers can more readily do some sync-mode testing on
    otherwise async-capable targets.

This requires a new doc review as the manual needed more changes.

Built and regtested on x86-64 Fedora 18, both in the ordinary mode and
using native-gdbserver; and on AIX.

Tom