Improve easter egg for 20 years of grml.org

Improve easter egg for 20 years of grml.org

Relevant changes:

1) Don't convert dates via date(1), but provide epoch seconds right away
2) Also use zsh's ${EPOCHSECONDS} instead of forking to date(1)
3) Display easter egg message with einfo iff we are within the birthday range
4) Don't display easter egg only on 2023-09-16 and one month later,
   but instead have the easter egg appear randomly, with diminishing probability as
   you get farther from the actual birthday
5) Don't display anything at all when booting with noeasteregg boot option

Thanks: Christopher Bock and András Korn