cleanups to copyuntil

[stevengj]

A couple of cleanups to #48273:

• Removed an overwritten method, as noted in copyuntil(out::IO, in::IO, delim) #48273 (comment) … I also rearranged the method definitions a bit to make them more readable.
• Made the ensureroom call in copyuntil(::IOBuffer, ::IOStream, ::UInt8) more conservative, as noted in copyuntil(out::IO, in::IO, delim) #48273 (comment)
• added tests for generic delim::T fallback methods of readuntil and copyuntil