change semantics of addrv62str
compress == 2 ==> full compression but doenst translate embedded IPv4
compress == 3 ==> full compression and translate embedded IPv4

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
make IPv6 compression level to 3

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
make st_prinf aware of compression level 3, and make it default

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
add help for -c option

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
Update README for IPv6 address compression

Signed-off-by: etienne basset <etienne.basset@numericable.fr>