memory: xmemcnt(data,c,len).
Refactor str_to_reg().

- Update the doxygen comments.
- Use more descriptive types.
- Localize variables.
- Find the '\n' with memchr instead of a for loop.
- Remove `if (size)` checks before memmove
  since memmove(dst,src,0) is a noop.
- Use memcpy instead since the pointers don't alias.
- Use xmemdupz instead of vim_strnsave.
- xrealloc instead of xmalloc/memcpy.
- Use xmemcnt.