Handle enable_event command with instrument target

Define a structure holds the event instrument target data. The last
data item is a flexible array that contains a variable number
of target path.

Add instrument target data to the prototype of cmd_enable_event(),
and process the instrument target data form clients in the function.

Add error codes of processing instrument traget data.

Conflicts:
	include/lttng/lttng-error.h
	src/bin/lttng-sessiond/main.c