Estimate maximal path length in skeleton and abort if it overflows.

Estimate maximal path length in skeleton and abort if it overflows.

Maximal skeleton path length is a bit different from YYMAXFILL:
it assumes that loops are iterated once (unlike YYMAXFILL calculation,
which disregards loops) and returns zero for empty regexp.

We need to know it in order:
    - to be sure it won't overflow
    - to store keys in a compact form (yet to be done)

This commit also makes DFA and skeleton store condition name and
source file line corresponding to current condition: it gets quite
annoying to pass these things around. This change caused another
change of test results (line numbers in error messages changed
for tests that use '-r' and reuse old DFA (don't reconstruct DFA
in 'use:re2c' blocks).