Allow to connect to a device using a connection string:

- Provide a nfc_get_default_device() that allow to grab the connstring stored
   in LIBNFC_DEFAULT_DEVICE environnement variable or returns the first
   available device if not set;
 - nfc_connect(NULL) now takes the default device (see
   nfc_get_default_device());
 - Removes nfc_driver_desc_t from public types
 - Defines nfc_connstring as char[1024]
 - examples/*: use nfc_connstring
 - examples/nfc-poll: now uses only the default device (instead of all devices
   availables)
 - Removes parse_args() from nfc-utils.[hc]

utils/nfc-utils.h

@@ -97,6 +97,4 @@ void    print_nfc_dep_info (const nfc_dep_info_t ndi, bool verbose);
 
 void    print_nfc_target (const nfc_target_t nt, bool verbose);
 
-nfc_device_desc_t *parse_args (int argc, const char *argv[], size_t * szFound, bool * verbose);
-
 #endif