Small fixes for incorrect error messages

Hi all,

I found a few easily overlooked error messages worth fixing, as follows:

01
```
if (!is_absolute_path(tsmap->new_dir))
- pg_fatal("old directory is not an absolute path in tablespace mapping: %s",
+ pg_fatal("new directory is not an absolute path in tablespace mapping: %s",
tsmap->new_dir);
```

02
```
if (rb < 0)
- pg_fatal("could not read from file \"%s\": %m", dst);
+ pg_fatal("could not read from file \"%s\": %m", src);
```

03
```
if (fstat(fd, &statbuf) < 0)
- pg_fatal("could not open file \"%s\" for reading: %m",
+ pg_fatal("could not stat file \"%s\" for reading: %m",
fullpath);
```

--
regards,
Man Zeng