the observed symptom was that the code was incorrectly rounding up
1.0625 to 1.063 despite the rounding mode being round-to-nearest with
ties broken by rounding to even last place. however, the code was just
not right in many respects, and i'm surprised it worked as well as it
did. this time i tested the values that end up in the variables round,
small, and the expression round+small, and all look good.

these should be tweaked according to testing. offhand i know 1000 is
too low and 5000 is likely to be sufficiently high. consider trying to
add futexes to file locking, too...

the previous fix was incorrect, as it would prevent f->close(f) from
being called if fflush(f) failed. i believe this was the original
motivation for using | rather than ||. so now let's just use a second
statement to constrain the order of function calls, and to back to
using |.

pcc turned up this bug by calling f->close(f) before fflush(f),
resulting in lost output and error on flush.

1. failed match of literal chars from the format string would always
return matching failure rather than input failure at eof, leading to
infinite loops in some programs.

2. unread of eof would wrongly adjust the character counts reported by
%n, yielding an off-by-one error.

don't waste time (and significant code size due to function call
overhead!) setting errno when the result of a syscall does not matter
or when it can't fail.

setting errno here is completely valid, but some programs, notably
busybox printf, assume that errno will not be set during output and
treat this as an error condition. in any case, skipping it slightly
reduces code size and saves time.