src/utils_tail.c: Call `stat' after reading till the end of file.

src/utils_tail.c: Call `stat' after reading till the end of file.

The old code used stat(2) to determine if the file was moved and reopened the
file before reading anything. This way lines, that were added to the file
before it was moved, would have been missed. This commit changes that behavior
so that the file is read until EOF and _then_ stat(2) is used to check if the
file has been moved away.