Some header files use __BEGIN_DECLS without including sys/cdefs.h.
This causes issues for C code that compiles against these headers,
which may occur with Ports.
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 *
