Add option to change the running directory before starting the process.
This can be done using -d or --chdir options. Add also test cases to
start-stop-daemon to test out the directory change option.

function                                             old     new   delta
packed_usage                                       34602   34648     +46
start_stop_daemon_main                              1107    1130     +23
start_stop_daemon_longopts                           156     164      +8
.rodata                                           105382  105384      +2
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(add/remove: 0/0 grow/shrink: 4/0 up/down: 79/0)               Total: 79 bytes

Signed-off-by: ejaaskel <esa.jaaskela@suomi24.fi>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>