gdbstub: Allow the %d placeholder in the socket path

gdbstub: Allow the %d placeholder in the socket path

Just like for QEMU_LOG_FILENAME, replace %d with PID in the GDB socket
path. This allows running multi-process applications with, e.g.,
export QEMU_GDB=/tmp/qemu-%d.sock. Currently this is not possible,
since the first process will cause the subsequent ones to fail due to
not being able to bind() the GDB socket.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20250117001542.8290-2-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250207153112.3939799-11-alex.bennee@linaro.org>