Start extension of name-value-[convert|get] to support deterministic
handling of repeated input paths. Implemented the template machinery, made
it all build (but the new stuff is not used yet).
Added command line options to use the new handling of repeated paths in
path-value. Builds and works in tested cases.
Alternative implementation for managing repeated paths in path-value input:
use search capabilities of the ptree itself, not extra containers. Unify all
template specializations for from_path_value_string.
Restore the old interface to from_name_value_string as a function. Pass the
checker type as the optional last argument (with no-check default). Adjusted
the applications to the new/old interface. Updated both -convert and -get.
Add a simple unit test for new options of name-value-convert.
Implement the second variant of path-value-to-ptree parser that appends
stuff to an existing ptree.
Update the expected file following the latest changes with quotes/no quotes
in name-value-eval-preparse.