While this code may be slightly less clear, it's significantly faster
and this function seems to be a hot path for certain workloads.

name                 old time/op    new time/op    delta
IsFQDN/no_dot-12       5.86ns ± 2%    1.48ns ± 3%  -74.71%  (p=0.000 n=10+10)
IsFQDN/unescaped-12    8.73ns ± 2%    1.57ns ± 1%  -81.98%  (p=0.000 n=9+8)
IsFQDN/escaped-12      27.4ns ± 2%    23.8ns ± 2%  -13.19%  (p=0.000 n=10+10)
FQDN/is_fqdn-12        8.36ns ± 1%    1.80ns ± 2%  -78.50%  (p=0.000 n=9+10)
FQDN/not_fqdn-12       36.8ns ±15%    33.4ns ±12%   -9.25%  (p=0.035 n=10+10)