Adjust some old text properties in ERC buffers

Adjust some old text properties in ERC buffers

* lisp/erc/erc.el (erc-display-message): Replace `rear-sticky' text
property, which has been around since 2002, with a more useful
`erc-command' property.  It records the current IRC command as a
symbol or a number, in the case of numerics.
(erc--own-property-names, erc--remove-text-properties) Add variable
and internal helper function for filtering values returned by
`filter-buffer-substring-function'.
(erc-display-prompt): Make the `field' text property more meaningful
to aid in searching, although this makes the `erc-prompt' property
somewhat redundant.
(erc-put-text-property, erc-list): Alias these to subr functions.
(erc-restore-text-properties): Don't forget tags when restoring.
(erc--get-eq-comparable-cmd): New function to extract commands for use
as more easily searchable text-property values.  (Bug#60936.)