This small function is intended as an easy way to mark all channels as
"read." This is useful when returning to an ERC session with multiple
modified channels that are not displayed in a window. Their status is
indicated in the mode line and in the ERC status sidebar, both of which
this function clears.

If this is a common enough task, it makes sense to bind this function to
a key sequence, e.g.:

  (keymap-global-set "C-c e c" 'erc-modified-channels-clear)

Note: I am not familiar with the ERC code base, so if calling the
function something else makes more sense (I originally called it
`erc-reset-modified-channels') I have no objection to it.