Update documentation

- Add several missing options to synopsis in ltrace.1
- Synchronize naming of arguments between ltrace.1 and --help
- Fix formatting in ltrace.1 -- argument names should be in emphasis,
  only the argument metavariables should.
Rename atomic singlestep to software singlestep

- The old name came from PPC implementation, where the feature was used for
  skipping atomic instructions.  But this is more useful, was reused for
  fully software singlestepping on MIPS, and will be similarly used for ARM.
  So rename the interface, and make the prototype more explicit.
- Also document the method in backend.h.
- And convert several void*'s to arch_addr_t's while there.
Update call to arch_sw_singlestep

- After the refactoring, the prototype changed and it doesn't
  return int anymore.  Of course C happily implicitly converts
  this.  Ho hum.
Move byte-reading routines from PPC backend to generic code

- Use backend hook umovebytes to read the bytes from inferior.