linux-user: fix /proc/self/stat handling

linux-user: fix /proc/self/stat handling

In the original bug report long files names in Guix caused
/proc/self/stat be truncated without the trailing ") " as specified in
proc manpage which says:
    (2) comm  %s
           The  filename of the executable, in parentheses.  This
           is visible whether or not the  executable  is  swapped
           out.

In the kernel this is currently done by do_task_stat calling
proc_task_name() which uses a structure limited by TASK_COMM_LEN (16).

Additionally it should only be reporting the executable name rather
than the full path. Fix both these failings while cleaning up the code
to use GString to build up the reported values. As the whole function
is cleaned up also adjust the white space to the current coding style.

Message-ID: <fb4c55fa-d539-67ee-c6c9-de8fb63c8488@inria.fr>
Reported-by: Brice Goglin <Brice.Goglin@inria.fr>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200414200631.12799-10-alex.bennee@linaro.org>