refactor interpl linked list to use std::unique_ptr

[rmu75]

continuing the work from #2486 this refactors NML_INTERP_LIST

• use std::unique_ptr instead of NMLmsg*
• adapt task to allocate message with std::make_unique

improvements:

• avoids multiple copying of message contents
• avoids some dynamic allocations in std::vector
• potentially saves heap space
• clear ownership / lifetime of message
• next step to more idiomatic use of standard library

[royka1]

@rmu75 This commit causes that the M67/M68 codes already get applied during the motion of the previous line, can you see where it goes wrong?

#2987

[rmu75]

@royka1 I will look into it, no idea at the moment.

[rmu75]

It seems I inadvertently removed some calls to flush_segments() in emccanon.cc -- I will prepare a patch.