Don't ignore rule symbol limits for full sentence rules

The full-sentence rules need to be as well-formed as any other.
The only difference is that they're allowed to ignore the max
span of initial phrase pairs for their left hand side.
Only use _S label for rules with long LHS spans

For very short training pairs, like dictionary entries, we don't
want to assign the (rather uninformative) label _S when something
better like a POS is available.

We reserve the special "_S" symbol for rules that could only be
extracted because the rule span limit is lifted for rules that
span the whole sentence.
Fix rule length limits for extracted rules

Case 1: hierarchical rules
In this case, extracted rules are not allowed to have more than
nonlex-{source,target}-length symbols on the right hand side.

Case 2: lexical rules
Here, the rules are restricted to the length of the initial phrase
pair. Initially, that happened automatically due to the way rules
are extracted, but now that the full sentence is a possible LHS,
we need to check for it explicitly.

NOTE: we're about to introduce a new configuration parameter that
will allow restricting the length of lexical rules further.
allow explicit selection of lexical phrase length limit

Before, it was limited to the initial phrase pair size, but in
practice, that's probably too long most of the time.

Introduced new conf keys: lex-{source,target}-words.
Pivoting properly runs, fixes to monotony-invariant target-conditioned features. Gzipped outputs.