Now we return user@host for USER() in embedded library
CURRENT_USER returns empty string if library compiled with
NO_EMBEDDED_ACCESS_CHECKS


libmysqld/embedded_priv.h:
  function's declarations trimmed
libmysqld/lib_sql.cc:
  user/host names handling added
libmysqld/libmysqld.c:
  user/host names handling added
sql/sql_class.cc:
  we shouldn't free user/host names in embedded library