fmt_ip4 formats an IPv4 number in dotted-decimal ASCII representation
from ip and writes the result into dest. It returns the
number of bytes written.
If dest equals FMT_LEN (i.e. is zero), fmt_ip4 returns the number
of bytes it would have written.
fmt_ip4 does not append \0.
For convenience, ip4.h defines the integer IP4_FMT to be big enough to
contain every possible fmt_ip4 output plus \0.