CLEANUP: general: get rid of all old occurrences of "session *t"

All the code inherited from version 1.1 still holds a lot ot sessions
called "t" because in 1.1 they were tasks. This naming is very annoying
and sometimes even confusing, for example in code involving tables.
Let's get rid of this once for all and before 1.5-final.

Nothing changed beyond just carefully renaming these variables.
3 files changed