Add support for file appends

Add option file_append (bool). If set, fio will append to a file
instead of operate within the size of it. This is similar to
setting offset= to the size of each file.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio.1 b/fio.1
index 20f0c09..fee5427 100644
--- a/fio.1
+++ b/fio.1
@@ -386,6 +386,12 @@
 that is given). If \fBfilesize\fR is not specified, each created file is the
 same size.
 .TP
+.BI file_append \fR=\fPbool
+Perform IO after the end of the file. Normally fio will operate within the
+size of a file. If this option is set, then fio will append to the file
+instead. This has identical behavior to setting \fRoffset\fP to the size
+of a file.
+.TP
 .BI blocksize \fR=\fPint[,int] "\fR,\fB bs" \fR=\fPint[,int]
 Block size for I/O units.  Default: 4k.  Values for reads, writes, and trims
 can be specified separately in the format \fIread\fR,\fIwrite\fR,\fItrim\fR