Extend the generator to generate a source (and the header for it) with
functions that print the content of a guestfs struct. The code is
modelled after the code for it in fish.ml, although made a bit more
generic (destination FILE*, line separator) so it can be used also in
the library, when tracing.
This just introduces the new code and builds it as part of the helper
libutils.la.
2c0d16e82e