Our coding style recommends C style comments (/* */) instead of C++
(//). Also, systemd (which we partly fork) uses C style comments for
the SPDX-License-Identifier.

Unify the style.

  $ sed -i '1 s#// SPDX-License-Identifier: \([^ ]\+\)$#/* SPDX-License-Identifier: \1 */#' -- $(git ls-files -- '*.[hc]' '*.[hc]pp')