detect: content limits propagation

detect: content limits propagation

Propagate inspection limits from anchered keywords to the rest of
a rule.

Examples:

content:"A"; depth:1; is anchored, it can only match in the first byte

content:"A"; depth:1; content:"BC"; distance:0; within:2;
"BC" can only be in the 2nd and 3rd byte of the payload. So effectively
it has an implicite offset of 1 and an implicit depth of 3.

content:"A"; depth:1; content:"BC"; distance:0; can assume offset:1; for
the 2nd content.

content:"A"; depth:1; pcre:"/B/R"; content:"C"; distance:0; can assume
at least offset:1; for content "C". We can't analyzer the pcre pattern
(yet), so we assume it matches with 0 bytes.

Add lots of test cases.