ptrace: move user_enable_single_step & co prototypes to linux/ptrace.h

While in theory user_enable_single_step/user_disable_single_step/
user_enable_blockstep could also be provided as an inline or macro there's
no good reason to do so, and having the prototype in one places keeps code
size and confusion down.

Roland said:

  The original thought there was that user_enable_single_step() et al
  might well be only an instruction or three on a sane machine (as if we
  have any of those!), and since there is only one call site inlining
  would be beneficial.  But I agree that there is no strong reason to care
  about inlining it.

  As to the arch changes, there is only one thought I'd add to the
  record.  It was always my thinking that for an arch where
  PTRACE_SINGLESTEP does text-modifying breakpoint insertion,
  user_enable_single_step() should not be provided.  That is,
  arch_has_single_step()=>true means that there is an arch facility with
  "pure" semantics that does not have any unexpected side e...