[ns] New option type for DSR

[Salvo Danilo Giuffrida]

Hello, I added a new flag (an int variable) to the hdr_sr class.
Should I then decrease by 1 the constant
MAX_SR_LEN
from 16 to 15, given the space occupied by this new flag, when it's
set? If it would be really implemented in DSR, would it occupy 4 bytes
(for alignment) or just 1 (actually, it's just a flag, it would need
only 1 bit)? In any case, should I care about this? I did some
measurements, and found out that with a value of 15, there is a slight
increase in the variance of some parameters across different
scenarios...