The problem is that if fread() returns a short count, we attempt
another fread() the next time through the loop, and apparently glibc
clears or ignores the eof condition so the second fread() requires
another ^D to make it see the eof condition.

According to the man page (and the C std, I hope) fread() can only
return a short count on error or eof.  I'm using that in the band-aid
solution to avoid calling fread() a second time after a short read.

Note that xreadlines() still has this problem: it calls
readlines(sizehint) until it gets a zero-length return.  Since
xreadlines() is mostly used for reading real files, I won't worry
about this until we get a bug report.