Return a string that is the name of no existing buffer based on NAME.
If there is no live buffer named NAME, then return NAME.
Otherwise modify name by appending `
(starting at 2) until an unused name is found, and then return that name.
Optional second argument IGNORE specifies a name that is okay to use (if
it is in the sequence to be tried) even if a buffer with that name exists.
If NAME begins with a space (i.e., a buffer that is not normally
visible to users), then if buffer NAME already exists a random number
is first appended to NAME, to speed up finding a non-existent buffer. (fn NAME &optional IGNORE)