Adopt linux' O_APPEND behavior

According to `man open`:

> The file is opened in append mode. Before each write(2), the file offset
is positioned at the end of the file, as if with lseek(2). The modi‐
fication of the file offset and the write operation are performed as a
single atomic step.
4 jobs from master in 1 minute and 17 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
failed #4114
build:linux:cargo

01:01

failed #4121
build:redox:cargo

00:15

failed #4115
build:redox:cargo

01:00

 
  Test
skipped #4116
test:linux:cargo
 
Name Stage Failure
failed
build:redox:cargo Build
error: Could not compile `spin`.

Caused by:
process didn't exit successfully: `rustc --crate-name spin /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/spin-0.4.6/src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="asm"' --cfg 'feature="const_fn"' --cfg 'feature="core_intrinsics"' --cfg 'feature="default"' --cfg 'feature="once"' --cfg 'feature="unstable"' -C metadata=1b0dffa6f445d302 -C extra-filename=-1b0dffa6f445d302 --out-dir /builds/jD91mZM2/redoxfs/target/x86_64-unknown-redox/debug/deps --target x86_64-unknown-redox -C linker=x86_64-unknown-redox-gcc -L dependency=/builds/jD91mZM2/redoxfs/target/x86_64-unknown-redox/debug/deps -L dependency=/builds/jD91mZM2/redoxfs/target/debug/deps --cap-lints allow` (exit code: 1)
warning: build failed, waiting for other jobs to finish...
error: build failed
groupadd: group 'redox' already exists
useradd: user 'redox' already exists
ERROR: Job failed: exit code 1
failed
build:linux:cargo Build
error: Could not compile `spin`.

Caused by:
process didn't exit successfully: `rustc --crate-name spin /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/spin-0.4.6/src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="asm"' --cfg 'feature="const_fn"' --cfg 'feature="core_intrinsics"' --cfg 'feature="default"' --cfg 'feature="once"' --cfg 'feature="unstable"' -C metadata=771bff99a36d0198 -C extra-filename=-771bff99a36d0198 --out-dir /builds/jD91mZM2/redoxfs/target/debug/deps -L dependency=/builds/jD91mZM2/redoxfs/target/debug/deps --cap-lints allow` (exit code: 1)
warning: build failed, waiting for other jobs to finish...
error: build failed
groupadd: group 'redox' already exists
useradd: user 'redox' already exists
ERROR: Job failed: exit code 1