apps: Fix O_ACCMODE bitmask checks after Linux flag alignment

[xiaoxiang781216]

Summary

Companion to apache/nuttx#19238 ("include/fcntl.h: align open flags with Linux values").

After aligning the NuttX open() flag constants with Linux (O_RDONLY=0, O_WRONLY=1, O_RDWR=2), code that used (flags & O_RDONLY) or (flags & O_WRONLY) as a bitmask check is broken because O_RDONLY is now 0. This PR fixes all such sites in apps/ by using (flags & O_ACCMODE) comparisons instead.

• usrsocktest (examples/usrsocktest/): replace flags & O_RDWR with flags & O_ACCMODE in fcntl(F_GETFL) assertions across 5 test files.
• dd (system/dd/dd_main.c): verify mode used (oflags & O_RDONLY) to detect verify; replace with (oflags & O_ACCMODE) == O_RDWR.
• dpopen (system/popen/dpopen.c): replace (oflag & O_RDWR) == O_RDWR with (oflag & O_ACCMODE) == O_RDWR.
• usbmsc (system/usbmsc/usbmsc_main.c): replace flags & O_WRONLY with (flags & O_ACCMODE) == O_RDONLY.
• fcntl_test (testing/testsuites/kernel/syscall/cases/fcntl_test.c): replace (flags & O_WRONLY) == 0 with (flags & O_ACCMODE) == O_RDONLY.

Impact

• Correctness: Without these fixes, the affected code silently misbehaves when O_RDONLY == 0 (e.g. dd verify never triggers, dpopen bidirectional detection fails, usbmsc read-only detection is wrong).
• Dependency: Must be merged together with (or after) !include/fcntl.h: align open flags with Linux values nuttx#19238. Standalone this PR has no effect since apache/nuttx still uses the old flag values.

Testing

Tested on the sim:nsh target (Linux x86_64 host, gcc) together with apache/nuttx#19238.

Boot log:

NuttShell (NSH) NuttX-10.4.0
nsh> uname -a
NuttX 10.4.0 6bb526f459a Jun 29 2026 23:37:06 sim sim
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK
 STACK COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0
069584 Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0
067456 sim_loop_wq
    2     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0
067472 hpwork
    4     4     0 100 FIFO     Task      - Running            0000000000000000 0
067496 nsh_main

OSTest: ran ostest from the NSH prompt — all tests passed with no failures or panics.