ftrace: add command interface for function selection

Allow for other tracers to add their own commands for function
selection. This interface gives a trace the ability to name a
command for function selection. Right now it is pretty limited
in what it offers, but this is a building step for more features.

The :mod: command is converted to this interface and also serves
as a template for other implementations.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>