Allow exemption from flood penalty in erc-backend

Allow exemption from flood penalty in erc-backend

* lisp/erc/erc-backend (erc-server-send, erc-server-PING): Change name
of param `forcep' in `erc-server-send' to `force' and change its type
to the union of the symbol `no-penalty' and the set of all other
non-nil values.  In `erc-server-PING', use this exemption when calling
`erc-server-send'.  This fix was fast tracked and summarily
incorporated into bug#48598 because users of the soju bouncer are all
affected.  See update #5 in the bug's email thread under the section
entitled "Riders" for an explanation.