values: make `bt_value_map_extend()` extend the provided base map

The current behavior of this function is to create a new extended map
leaving the two provided maps untouched. I can see cases where that
would be useful, but I believe the user is going to expect the
`_extend()` function to extend an existing map.

If a user want to get a new map, she could still copy the base map using
`bt_value_copy()` before extending it.

So this commit changes the behavior of `bt_value_map_extend()` so that
it takes two arguments: the base map, and the extension map. The entries
of the extension map are added to the base map, overwriting any existing
keys.

Also, remove the `_MEMORY_ERROR` status from the
`bt_value_map_foreach_entry_status` and
`bt_value_map_foreach_entry_const_status` enums since those two are not
used at the moment.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I8fa7d4591da90dae50316d6db86af5ab1b76203c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2162
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>