Move the q_append / qs_append / write_at_position functions away
from the String class and into free functions. This allows
sql_string.h not to include my_byteorder.h anymore. Also, they are
unsafe, have few users, and should not be in the general interface
of a string class.

Change-Id: I211e792dc097f428065f42661d8e9069bc54f4b1