pidl: add a base class for PIDL parsers

pidl: add a base class for PIDL parsers

There are about 5 object-oriented parsers, all with their own
effectively identical but differently spelt versions of pidl(),
pidl_hdr(), indent(), and deindent(). With this commit we add a base
class that they can all use.

The ultimate aim is to be able to add some debugging instrumentation
that benefits all[1] the parsers.

[1] The parsers (e.g. Samba::ServerNDR) which use global scope rather
than objects will not be affected.

The versions of the functions in this file follow the most
sophisticated versions of the soon-to-be subclasses. For example, the
pidl() function avoids spurious whitespace and puts #define at column
0, following the Python parser.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>