regcomp.c - cleanup the ahocorasick start class logic so it more self-documenting

regcomp.c - cleanup the ahocorasick start class logic so it more self-documenting

The logic of setting up an AHO-CORASICK regex start class was not fully
encapsuated in the make_trie_failtable() function, which itself was
poorly named. Merged the code into make_trie_failtable() and renamed
it to construct_ahocorasick_from_trie().