PR14650: combine more userspace-deref functions

PR14650: combine more userspace-deref functions

* runtime/linux/copy.c: Move out _stp_read_address definition.
  (__stp_strncpy_from_user): Simply accept vicarious protection
  from caller _stp_strncpy_from_user.
  (_stp_copy_from_user): Protect more.
* runtime/stp_string.c (_stp_text_str): Use _stp_read_address
  instead of barenaked __stp_get_user.
* runtime/stp_string.h (__stp_get_user): Simplify; now only for
  use by ...
  (_stp_read_address): Moved here.