Oh no, 3413d1bf was broken!  Instead of passing a number as
"seconds" to the duration constructor, it just abused the duration
constructor as cast operator, which caused custom state_file_interval
settings to be extremely short.

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

Move code from config_global_finish().

Now config_param::GetBlockValue() can be used to get signed integers
from the configuration.

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.

Replaces GLib's GError.