SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
The return value is always be 'count', even in the case of 0.
Note that the return value of TypedTransfer::copy() is likewise uninteresting,
but apparently it is beig used. Hence this patch does not touch it.
