Enable discard support for the null target unconditionally. The null
device now advertises discard capabilities and handles discard/write-zeroes
commands by returning a success status (0), as no actual data operation
is needed.

Also, add a basic integration test to verify that the `blkdiscard` command
runs successfully against a `rublk` null device.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>

Create a single `has_blkdiscard()` helper function to check for the
availability of the `blkdiscard` command.

This refactoring removes code duplication from the `test_ublk_loop_discard`
and `test_ublk_null_discard` tests, making the test suite cleaner and
easier to maintain.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>

This commit refactors `src/args.rs` to improve code clarity, robustness,
and adherence to Rust idioms.

- Replaced `.unwrap()` calls with `?` to ensure proper error propagation
when building the `UblkCtrl` object.

- Simplified boolean flag handling for setting control flags.

- Added comments to clarify the purpose of the `start_dir` field and the
`round_up` and `is_power2_of` helper functions.

- Improved the implementation of `is_power2_of` to use the standard library's
`is_power_of_two()` for better readability.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>

When `rublk add` is run in a subshell, as in `$(rublk add null)`, the parent
shell waits for the daemon's stdout and stderr file descriptors to close.
Previously, these were kept open, causing the parent to hang indefinitely.

This commit fixes the issue by redirecting the daemon's stdout and stderr to
`/dev/null`. This allows the daemon to fully detach, preventing the parent
shell from hanging and enabling correct behavior in subshells.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>