[doco] Document the XML format used for AIX shared lib support.

gdb/doc/ChangeLog:

        * gdbint.texinfo (Native Debugging): Add "AIX Shared Library
        Support" subsection documenting the XML format used to transfer
        shared library info on AIX.
Dummy implementation of solib_aix_free_library_list if libexpat missing.

gdb/ChangeLog:

        * solib-aix.c (solib_aix_free_library_list): New function
        for the case where HAVE_LIBEXPAT is not defined.
ti.ti_pc vs ti.ti_startfunc copy-pasto in sol-thread.c:info_cb

gdb/ChangeLog:

        * sol-thread.c (info_cb) [ti.ti_state == TD_THR_SLEEP]:
        Replace ti.ti_startfunc by ti.ti_pc.

Probably OK to commit without approval, so will likely do so in the
next few days.
Improve output of "maintenance info sol-threads" command.

This patch does the following:
  - Puts the startfunc and "Sleep func" info on the same line;
  - Renames "Sleep func" into "sleepfunc" to be consistent with
    "startfunc"
  - Avoids the use of a '-' as a separate before the "sleepfunc"
    output, because the '-' looks odd and out of place when the
    "startfunc" field is not printed (ti.ti_startfunc is nul).
  - Use a '=' instead of ':' and avoids the space between
    the name of the value and its value, mostly to help group
    the value with its name.

For the record, this is how the new outout now looks like:

    (gdb) maintenance info sol-threads
    user   thread #1, lwp 1, (active)
    system thread #2, lwp 2, (active)
    system thread #3, lwp 0, (asleep)    sleepfunc=0xff32d9e0
    user   thread #4, lwp 4, (asleep)    startfunc=[...].task_wrapper   sleepfunc=0xff3290f0
    system thread #5, lwp 7, (active)    startfunc=_co_timerset
    user   thread #6, lwp 8, (active)    startfunc=[...].task_wrapper

gdb/ChangeLog:

        * sol-thread.c (info_cb): Rework the output of the "maintenance
        info sol-threads" command a bit.
Avoid duplicating text in sol-thread.c:info_cb

This is a minor cleanup that helps avoiding duplicating the output
of a command when handling alternative situations.

gdb/ChangeLog:

        * sol-thread.c (info_cb): Factorize the code a little.