Return event fields by a field name

Since a field may be present both in an event and its stream's scope, this returns
a list which contains all the fileds by the name. The examples are also changed
according to this API.
Remove the unnecessary underscore prefix

When getting the fields of an event by a name, there is no necessary
to add an underscore prefix. This removes the underscore prefix of
the field name in the examples.
Support getting the value of enums

Use 'get_enmu_str' to get the string matching the current enumeration.
Use a unique name to get a field's value

Use get_value to get a field's value. Currently it does not support
compound types. Also it does not support floating point, since getting
a float's value is not implemented in babeltrace now.