file-source: support incremental read from files with preallocated size

I work with a system that preallocates 4MB file for log and fill it with all zeros.

00014770: 69 74 20 20 20 3A 20 74 72 61 6E 73 61 63 74 69    it...:.transacti
00014780: 6F 6E 20 63 65 6E 74 65 72 20 64 65 6C 65 74 65    on.center.delete
00014790: 64 0D 0A 30 09 31 36 3A 32 33 3A 33 30 2E 30 38    d..0.16:23:30.08
000147a0: 32 09 09 45 78 69 74 20 20 20 3A 20 61 63 74 69    2..Exit...:.acti
000147b0: 76 69 74 79 20 63 65 6E 74 65 72 20 64 65 6C 65    vity.center.dele
000147c0: 74 65 64 0D 0A 00 00 00 00 00 00 00 00 00 00 00    ted.............
000147d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
...

Here you can see last line termination bytes 0D 0A \r\n and then zeros bytes. Now vector tries to read the line of all zeros, discard long line and then don't try to parse the line again.

It would be great to use vector to parse that kind of files.

No source file or test is named. Start by tracing the file-source entry point and its line-reading behavior for preallocated zero-filled files; done means trailing zero bytes are not treated as a permanently discarded long line and later appended log data is parsed.