Fixed bug #60 "redundant use of YYMARKER".

Fixed bug #60 "redundant use of YYMARKER".

Bug description: sometimes re2c would generate code that backups
current input position (e.g. 'YYMARKER = YYCURSOR'), but wouldn't
generate code that restores backuped position (e.g. 'YYCURSOR =
YYMARKER').

Analyses: DFA may have overlapping rules (e.g. "a" and "aaa").
In such cases, if the shorter rule matched, lexer must attempt to
match the longer one. If the longer rule also mathed, then lexer
prefers it to the shorter rule. If the longer rule didn't match,
lexer must backtrack input position to the point when the shorter
rule matched. In order to be able to backtrack, re2c must generate
backup code (e.g. 'YYMARKER = YYCURSOR') and restore code (e.g.
'YYCURSOR = YYMARKER').
In some rare cases DFA has overlapping rules, but if the shorter rule
matched, then the longer rule will always match (perhaps on an
arbitrary long input string), e.g.:
    /*!re2c
        [^]+ "a" { 1st }
        "b"      { 2nd }
    */
In this cases there's no need to generate backup code for 2nd rule:
lexer will either encounter final "a" and the 1st rule will match
or YYFILL will not return; anyway, restore code will never be run.
re2c used to output backup code but not restore code in such cases.
This is the bug: backup code is useless without restore code and
should be omitted.

In future re2c should warn about such cases (when the shorter of
two overlapping rules is shadowed by the longer one).

The fix: postpone insertion of save actions (those with backup code)
untill it is known if restore code will be generated.
I also removed obsolete global variable 'bUsedYYMarker', which was
always set to 'true' (it should be per-DFA, not per-block configuration
anyway).