Move 7 different functions for allocating LEX_STRING/LEX_CSTRING
from sql_class.h to strfunc.h where they more naturally belong.

Simplify them by removing unneccessary arguments and combine them
so that we end up with just 3 functions.

Improve code documentation.

Change-Id: Icf0dc0ef944435eafcd737171411b76ad1f61698