uci: fix memory leak uci_set on update section

uci: fix memory leak uci_set on update section

If uci_realloc fails when updating a section in uci_set the reference
to the memory allocated by s = uci_strdup() is lost.

Also, if uci_strdup and uci_realloc both succeed it could happen that
ptr->s->type == uci_dataptr(ptr->s) by accident. Then later on in
uci_free_section the allocated ptr->s->type is not freed.

In order to fix this, instead of splitting the allocation of the the
section and the type string, we create a new section with in-section
storage to replace the old one. This also brings the code for updating
a section more in line with the simular code for updating an option.

Signed-off-by: Jan Venekamp <jan@venekamp.net>