Adjusted file->sys_offset after the write() syscall.

This fixes suboptimal behavior caused by surplus lseek() for sequential writes
on systems without pwrite().  A consecutive read after write might result in an
error on systems without pread() and pwrite().

Fortunately, at the moment there are no widely used systems without these
syscalls.

src/os/unix/ngx_files.c

diff --git a/src/os/unix/ngx_files.c b/src/os/unix/ngx_files.c
index 417b598..36b6b67 100644
--- a/src/os/unix/ngx_files.c
+++ b/src/os/unix/ngx_files.c
@@ -226,6 +226,7 @@
             return NGX_ERROR;
         }
 
+        file->sys_offset += n;
         file->offset += n;
         written += n;