main,lregex: revise the way to using group numbers of regex matching

[masatake]

Inspired from #2598.

The lregex code uses group numbers of regex matching in many places. {mgroup=N} is one of them

I have found many issues about usage.

• [ ] a default value should be given to mgroup. ...

[masatake]

I wonder what happens if we give --_mtable-regex-Foo=main/([a-z]*)/\1//{mgroup=9}. In many places, the range check of the regex group number is missing.