func: factor out func_def_new and func_def_delete

func_def_new takes function id, name, body, comment, and owner id and
allocates a new func_def struct, setting the rest of the members to
their default values. We need this function to create a new func_def
object for handling space upgrade in read view.

Note, this isn't a pure refactoring - before this patch, we used
FUNC_LANGUAGE_LUA for SQL builtin functions, which were deprecated in
2.9. This worked fine, because we never actually called them - it was
needed solely for upgrade from older versions. In this commit, we create
an SQL builtin function just like any other function, but set its vtab
to a dummy, which raises an error on an attempt to call it. This should
make the code clearer.

Needed for https://github.com/tarantool/tarantool-ee/issues/163

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring