fileio: fix read_full_stream() bugs (#3887)

read_full_stream() _always_ allocated twice the memory needed, due to
only breaking the realloc() && fread() loop when fread() returned 0,
requiring another iteration and exponentially enlarged buffer just to
discover the EOF condition.

This also caused file sizes >2MiB && <= 4MiB to erroneously be treated
as E2BIG, due to the inappropriately doubled buffer size exceeding
4*1024*1024.

Also made the 4*1024*1024 magic number a READ_FULL_BYTES_MAX constant.