Prepare parasite_infect_seized() for compel-ization

So, this is the main preparatory set to split the infection code
into compel. The biggest challenge at this point is to teach the
code that gets tasks registers and later puts them on sigframe
to keep these registers in CRIU-independent format.

Since generic compel user will likely not want to mess with the
registers himself, the proposal is to make the registers keeper
"pluggable", i.e. -- after creating the parasite ctl set up custom
callbacks on save/restore regs. For CRIU this will be the functions
that keep regs on CoreEntry, for generic compel users these
calls will be set by compel defaults that would just memcpy() them.

So here's the set that makes this generalization and splits the
parasite_infect_seized() into two parts.

Set applies on top of recently sent user_fpregs_struct_t ones.

-- Pavel