improve creation of directories for destination files

[adammoody]

AXL currently creates directories for each destination file. However, there are several things that could be improved.

First, the destination directories may already exist, in which case it's wasteful workload to ask the file system to create them again. Directory creation should be optional. There should be a way to configure whether dirs are created via the API. For example, we could define a new AXL_Mkdir call to be explicit, or flags could be added to AXL_Create or AXL_Add.

Second, it's not clear what permissions/group should be assigned to the newly created directory. Another vote for AXL_Mkdir.

[adammoody]

For example, Filo creates directories before it invokes AXL, so it would be helpful if Filo could disable the directory creation in AXL:
https://github.com/ECP-VeloC/filo/blob/f7ce43e1838b558b2db1a9c6441d7595524c682f/src/filo.c#L848