Add eventual replacement for erc-default-recipients

Add eventual replacement for erc-default-recipients

* lisp/erc/erc.el (erc--target, erc--target-channel,
erc--target-channel-local): Add new structs to hold info on a buffer's
target; stored in a local variable of the same name.
(erc--target-from-string): Add standalone constructor for
`erc--target'.
(erc--default-target): Add temporary internal getter to ease
transition to `erc--target' everywhere.
(erc-open): Create above items in non-server buffers.

* lisp/erc/erc-backend.el (erc-server-NICK): Recreate `erc--target'
when necessary.