Rename syscall_buffer.h to preload_interface.h.
Make rrcall_init_buffers_params a per-Arch struct when compiling its definition in rr.
Make syscallbuf_record::ret 64 bits on all architectures.
Move socketcall_args out of preload_interface to AutoRemoteSyscalls.
Replace explicit syscall instruction data with helper functions is_at_syscall_instruction, syscall_instruction, and syscall_instruction_length.
Template retrieve_fd over Arch.
Add sockaddr_un and cmsg definitions to Arch.
Rewrite retrieve_fd code to handle variable-Arch children.

Also generally cleans up that code a lot.