Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
Rename actual main to `${tool_name}_main` and have a proxy main call it.
In the next patch we will get rid of these proxy mains and the tool
mains will be invoked from scylla's main, if the `argv[0]` matches their
name.
The main functions are included in a new `tools/entry_point.hh` header.
