liburcu: correct licenses

liburcu: correct licenses

I've listed what files contain each SPDX-License-Identifier, and tried
to classify their purpose as below:

- BSD-2-Clause: `tests/**` (tests)
- CC0-1.0: `extras/abi/**/*.xml` (documentation)
- CC-BY-4.0: `ChangeLog`, `**/*.md` (documentation)
- FSFAP: `m4/*.m4` (build system)
- GPL-2.0-only: `tests/**`, `extras/abi/dump_abi.sh` (tests,
  documentation)
- GPL-2.0-or-later: `scripts/urcu-api-list.sh`, `tests/**` (tests,
  documentation)
- GPL-2.0-or-later WITH Autoconf-exception-2.0: `m4/ae_pprint.m4` (build
  system)
- GPL-2.0-or-later WITH LicenseRef-Autoconf-exception-macro: `m4/*.m4`
  (build system)
- GPL-3.0-or-later: `tests/utils/tap.sh` (tests)
- LGPL-2.1-only: `configure.ac`, `include/**/*.h` (build system,
  headers)
- LGPL-2.1-or-later: `doc/**`, `include/**/*.h`, `src/**/*.{c,h}`,
  `tests/**`, (documentation, headers, source, tests)
- LicenseRef-Boehm-GC: `include/**/*.h`, `tests/common/thread-id.h`
  (headers, documentation)
- MIT: `.gitignore`, `.gitreview`, `Makefile.am`, `bootstrap`, `doc/**`,
  `extras/Makefile.am`, `include/Makefile.am`, `include/**/*.h`,
  `src/**/*.{am,h,pc.in}`, `tests/**` (documentation, build system,
  headers, source control)

Then in PKG_LICENSE I've included licenses which are used for headers,
source, or build system. I've also corrected PKG_LICENSE_FILES: license
texts are in the LICENSES directory, lgpl-relicensing ends with .md, and
I've added the overarching LICENSE.md.

Signed-off-by: Linus Kardell <[email protected]>