Send enable-event commands with instrument target

Create a static function enable_event() to deal with all possible
permutations of data that accompany with the enable-event command:
- all events / some events
- no filter / a filter
- no exclusions / some exclusions
- no instrument target / a instrument target

Wrap existing functions lttng_enable_event(),
lttng_enable_event_with_filter() and lttng_enable_event_with_exclusions()
with the new static enable_event() functions without breaking API.

Create a new lttng_enable_event_with_target() function to handle the
instrument target data. Note that it is not possible to enable events
with excusions together with instrument target, since exclusions only
work with wildcard events.
Parse and forward instument probes

Report error if userspace probes created with wildcard events.
Add usage for userspace instrumented probes
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.