Decouple disparate escape-hatch concerns in erc-stamp

Decouple disparate escape-hatch concerns in erc-stamp

* lisp/erc/erc-stamp.el (erc-stamp--allow-unmanaged): Improve doc
string.
(erc-stamp--permanent-cursor-sensor-functions): New variable to take
over the formerly provided `cursor-sensor-functions' aspect of the
flag `erc-stamp--allow-unmanaged'.
(erc-add-timestamp): Use
`erc-stamp--permanent-cursor-sensor-functions' instead of
`erc-stamp--allow-unmanaged' in guard condition.
(erc-munge-invisibility-spec): Use dedicated compatibility flag
`erc-stamp--permanent-cursor-sensor-functions' and forgo unnecessary
setup when it's non-nil.  (Bug#60936)