This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Move code from config_parse_path().

The old struct config_param remains only for top-level string options.

Move code from config_global_finish().

Merge duplicate code.

Fixes building with gcc-4.8
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.