detect/app-layer-proto: don't run detection on ALPROTO_UNKNOWN

detect/app-layer-proto: don't run detection on ALPROTO_UNKNOWN

The `app-layer-protocol` keyword inconsistently checks whether the
alproto is ALPROTO_UNKNOWN. In the regular match function it isn't
checked, in the prefilter function its checked for all but the "either"
mode.

This leads to false positives for negated matching, as an expression
like "!tls" will match if checked against ALPROTO_UNKNOWN.

This patch adds the checking everywhere. The keyword returns no match as
long as the alproto is ALPROTO_UNKNOWN.

Bug: #7241.