Fix bunch of warnings about GNU extension under clang.

Clang complaints are like:
./dvblast.h:136:22: warning: field 'raw_pkt_header' with variable sized type
      'struct udprawpkt' not at the end of a struct or class is a GNU extension
      [-Wgnu]
    struct udprawpkt raw_pkt_header;
                     ^
Fix enumeration value not handeled in switch.

SYS_DMBTH was introduced in DVB API version 5.0.0, and deprecated in favor
of SYS_DTMB in DVB API version 5.0.7.