add -Werror to CFLAGS
add -Wno-error=sign-conversion to CFLAGS of libruby
fix compiler warnings
fix CFLAGS of libruby
fix set-vlan byteorder

bytes in a frame are stored in network byte order.
vlan priority bit shifts was wrong.
Merge branches 'fix_set_vlan' and 'fix_set_vlan' of https://github.com/iHiroakiKawai/trema-edge into fix_set_vlan