pg_upgrade bugs

I have been wrestling for a couple of days trying to get pg_upgrade
testing working on Windows, with a view to having it tested on the
buildfarm. The test script has its own issues, which I'll deal with
separately, but there are two issues in pg_upgrade's exec.c that make me
suspect that if pg_upgrade has ever worked at all on Windows it is a
matter of sheer luck. The attached patch fixes these. The first issue is
a plain miscall to stlcpy(), where the length argument is wrong. The
second is where exec_prog tries to open a log file after the system call
returns. This will fail if the command was a 'pg_ctl start', as the
running postmaster will have the log file open, so I have simply
#ifdef'd it out for the Windows case, as the code does nothing except
add a couple of line feeds to the log, missing which won't affect
anything much.

Barring objection I will commit this before long.

cheers

andrew