services_discovery: add SD_CMD_NAVIGATE
dsm: refactorize

Move browser.c to common.c. This new file contains common browsing functions
that can be used from an access_t or from a services_discovery_t.
dsm: only one function to browse share and directory
dsm: don't export too common functions
dsm: add "smb://" if not present
dsm/sd: discover in a separate thread
dsm/sd: thread stay alive and handle commands