<sys/unistd.h>: Rename struct lwp_params member names.

<sys/unistd.h>: Rename struct lwp_params member names.

This is mainly to better "protect" them against 3rd party software doing
silly things like lvm2 which has a "#define stack ..." that conflicted
with the struct's 'stack' member, for example. It's a lame reason to
rename the struct members, but, given that it is a public struct, it's
a good thing to do so generally.

Also, because it is a public structure, bump __DragonFly_version so it
can be dealt with in code. I didn't find any usage of our lwp_* code
in dports but it might be used in someone's own code still.