allow the 'printing' parameter to be set on a per share basis.

allow the 'printing' parameter to be set on a per share basis.
The problem was that the current_printif struct was set during
print_backend_init() based on the 'printcap name'.  So you could
not use cups and then override the setting for a specific printer
by setting 'printing = bsd' (a common setup for pdf generation
print services.

There is a subtle change in behavior in that the print
interface functions are selecting on the basis of lp_printing()
and not lp_printcap_name(), but the new behavior seems more
intuitive IMHO.