stap: unite all dumping modes into s.dump_mode

stap: unite all dumping modes into s.dump_mode

Clean up the way dumping modes are implement. We create a new enum
member which tracks the type of dumping wanted. This enum handles all
stap invocations which do not directly handle a user-script:
-l/-L/--dump-probe-types/--dump-probe-aliases/--dump-functions. It also
allows us to clean up the cmdline_script/have_script hacks previously
used in switch handling.

The trickiest part about this patch is to now allow for the possibility
of s.user_file to be NULL throughout passes 1 and 2, which previously
always assumed a script was present.