Pass unquoted strings to parsing functions.

Pass unquoted strings to parsing functions.

In most cases re2c accepts single-quoted or double-quoted strings
in regexp specifications, but if flex-like syntax if enabled, then
re2c accepts unquoted strings.

Before this commit, functions that parse strings into regexps
expected quoted strings, and we had to add quotes in case of
flex-like syntax. That was very inconvenient and in fact
unnecessary, since the first thing parsing functions do is get rid
of quotes.