readbytes!: support the all keyword for all methods

[StefanKarpinski]

It is quite surprising that the readbytes! function doesn't block to fill up its buffer argument by default. Fortunately, there's a all=true keyword argument that you can pass to it to make it do so. Unfortunately, that keyword argument isn't supported by all I/O types so you still end up having to implement this logic yourself if you want to use readbytes! generically. The all keyword argument should be extended to cover all methods.

[vtjnash]

all=true is the default (consuming the whole stream), but you can pass all=false to only block once

[StefanKarpinski]

I've encountered cases where it doesn't seems block. Will try to come up with an example.

[notinaboat]

The generic readbytes! spec is "at most nb bytes" without an all= option.

julia/base/io.jl

Lines 955 to 962 in c298e4e

	"""
	readbytes!(stream::IO, b::AbstractVector{UInt8}, nb=length(b))
	Read at most `nb` bytes from `stream` into `b`, returning the number of bytes read.
	The size of `b` will be increased if needed (i.e. if `nb` is greater than `length(b)`
	and enough bytes could be read), but it will never be decreased.
	"""
	function readbytes!(s::IO, b::AbstractArray{UInt8}, nb=length(b))

Essentially the same issue as #17070 ?