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 *
Semaphores with values greater than one didn't work because whoever
called sem_wait() first held the semaphore's mutex until a matching
sem_post() call.
Other callers then wouldn't be able to acquire the semaphore even
if the semaphore's value was still greater than zero at that point.
