[x265] --nal-hrd cli option

[dave]

In x264, NalHrdParametersPresentFlag can be set on the command line with 
the --nal-hrd option with possible arguments of none, vbr and cbr.  
Currently x265 sets it based on bitRate > 0.  I copied the --nal-hrd 
option from x264 into x265 but it only sets m_hrdParametersPresentFlag.  
I left m_nalHrdParametersPresentFlag alone because it was set 
elsewhere.  If that is preferred then perhaps --nal-hrd could be more 
accurately named --hrd or I can update x265 to work with 
m_nalHrdParametersPresentFlag like x264.