The dysfunctional APT is apparently a consequence of me using
ppa:ubuntu-toolchain-r/test:
https://askubuntu.com/questions/777803/apt-relocation-error-version-glibcxx-3-4-21-not-defined-in-file-libstdc-so-6
I had that PPA (and all other external repository) disabled before the
upgrade but I believe I should have removed all the packages coming from it.
This morning I discovered that the versions 4.9.389-gnu, 4.9.40-gnu,
4.14.10-gnu (all from jxself's Linux-libre repository), 4.4.0-104 (Trisquel
8's default) or 3.13.0-137 (Trisquel 7's default) of the kernel would all
"panic", even in "recovery mode". So, yes, it does not look I have options
(such as those suggested in the page I linked above) but installing from an
ISO.
A warning for those using the "Ubuntu toolchain PPA" would be good in
Trisquel 8's release notes, I guess.
