tracetool: handle DTrace keywords 'in', 'next', 'self'

Language keywords cannot be used as argument names.  The DTrace backend
appends an underscore to the argument name in order to make the argument
name legal.

This patch adds 'in', 'next', and 'self' keywords to dtrace.py.

Also drop the unnecessary argument name lstrip() call.  The
Arguments.build() method already ensures there is no space around
argument names.  Furthermore it is misleading to do the lstrip() *after*
checking against keywords because the keyword check would not match if
spaces were in the name.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Alon Levy <alevy@redhat.com>
Reviewed-by: Lluís Vilanova <vilanova@ac.upc.edu>
