Allow application to set ALSA devices manually

[nanangizz]

Currently the library will automatically enumerate ALSA devices. However, application may want to have access to the subdevices. The new API pjmedia_aud_alsa_set_devices() will allow application to manually set custom ALSA devices, e.g:

#include<pjmedia-audiodev/alsa.h>
...
// Set ALSA devices manually
const char* custom_dev_names[] = {"dev1", "dev2"};
status = pjmedia_aud_alsa_set_devices(NULL, 2, custom_dev_names);

// Let the driver to enumerate the devices
status = pjmedia_aud_alsa_set_devices(NULL, 0, NULL);