Out-of-bounds write and incorrect detection of trigger file in pg_standby

Hi all,

In pg_standby.c, we use a 32-byte buffer in CheckForExternalTrigger to
which is read the content of the trigger file defined by -f:
CheckForExternalTrigger(void)
{
    char            buf[32];
[...]
    if ((len = read(fd, buf, sizeof(buf))) < 0)
    {
        stuff();
    }

    if (len < sizeof(buf))
        buf[len] = '\0';
However it happens that if the file read contains 32 bytes or more, we
enforce \0 in a position out of bounds. While looking at that, I also
noticed that pg_standby can trigger a failover as long as the
beginning of buf matches either "smart" or "fast", but I think we
should check for a perfect match instead of a comparison of the first
bytes, no?

Attached is a patch to fix the out-of-bound issue as well as
improvements for the detection of the trigger file content. I think
that the out-of-bound fix should be backpatched, while the
improvements for the trigger file are master-only. Coverity has
pointed out the out-of-bound issue.
Regards,
--
Michael