Allow spoofing process marker in erc-display-line-1

Allow spoofing process marker in erc-display-line-1

* lisp/erc/erc.el (erc--insert-marker): New internal variable for
overriding `erc-insert-marker' when displaying messages at a
non-default location in the buffer.
(erc-display-line-1): Favor `erc--insert-marker' over
`erc-insert-marker' when non-nil.
; * test/lisp/erc/resources/base/assoc/multi-net/barnet.eld: Timeouts.
; * test/lisp/erc/resources/base/assoc/multi-net/foonet.eld: Timeouts.
; * test/lisp/erc/resources/base/netid/bouncer/barnet-drop.eld: Timeouts.
; * test/lisp/erc/resources/base/netid/bouncer/foonet-drop.eld: Timeouts.
; * test/lisp/erc/resources/erc-d/resources/dynamic-foonet.eld: Timeouts.
; * test/lisp/erc/resources/sasl/scram-sha-1.eld: Timeouts.
; * test/lisp/erc/resources/sasl/scram-sha-256.eld: Timeouts.