current logic how the quotes (both ' and ") are managed is a bit too relaxed,
allowing wierd constructs like set name="value' also usual single quote
semantics is not possible and, the code does not check if the quoted string
actually has ending quote.

This update implements:

1. distinguishing single and double quote
2. variable expansion will not be done inside single quote protected area
3. will preserve inner quote for values like "value 'some list'"
4. ending quote check.

however, this diff does not implement ending quote order check - it shouldn't be too hard, needs some improvements on parser state machine.

this is fix for bug: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=204602