update license data and convert to SPDX format
    
    The license data was gathered from the 2.38.1 tarball.  The licensecheck
    tool was run:
    
        find -type f -regextype egrep ! -regex '^(Documentation/.*\.txt$|(t/(chainlint|perf/p[0-9]{4}|t[0-9]{4}).*))' \
            -exec licensecheck --shortname-scheme spdx {} + | LANG=C sort >licensecheck
    
    The contents were reviewed, removing files which are not shipped or were
    UNKNOWN to licensecheck.  Of the UNKNOWN files, most lacked a specific
    license header and are thus treated as GPL-2.0-only.  The code in
    reftable/ is licensed as BSD 3-Clause per reftable/LICENSE.