sched/task: MISRA/Coverity cleanups in scheduler, init, libc spawn, and ARMv8-R IRQ helpers

[fxysunshine]

Summary

This PR addresses a small set of MISRA C:2012 and Coverity findings across NuttX core code.
The changes are focused on type-safety, initialization, and simplifying return paths without altering intended behavior.

Key updates:

• Fix MISRA Rule 10.4 by using the proper enum type for g_nx_initstate.
• Fix MISRA Rule 10.3 in ARMv8-R IRQ helpers by using unsigned constants (1u, ~1u) for bit operations.
• Fix a Coverity uninitialized-field report by zero-initializing posix_spawnattr_t in posix_spawnattr_init().
• Refactor several scheduler/task helpers to use a single return path (Coverity “RETURN” metric), improving readability and reducing early returns.

Impact

• Functional impact: No intended functional change; these are compliance/quality fixes (type correctness, initialization, control-flow cleanup).
• Affected areas:
  • Scheduler/task utilities (sched/task/*)
  • System init state (include/nuttx/init.h, sched/init/nx_start.c)
  • libc spawn attributes (libs/libc/spawn/lib_psa_init.c)
  • ARMv8-R IRQ helpers (arch/arm/include/armv8-r/irq.h)
• Risk: Low. Changes are localized and primarily mechanical (types, constants, return-path refactors).

Testing

• Environment:
  • Host: Linux
  • Build system: CMake
  • Configurations:
  • sim:nsh and qemu-armv7a:nsh
• Steps:

1. Configure and build sim:nsh (CMake):
   • cmake -B build -DBOARD_CONFIG=sim:nsh -GNinja
   • cmake --build build -j
2. Configure and build qemu-armv7a:nsh (CMake):
   • cmake -B build -DBOARD_CONFIG=qemu-armv7a:nsh -GNinja
   • cmake --build build -j
3. Boot and run NSH smoke checks:
   • Run help
   • Run ps ...

• Results:
  • MISRA/Coverity: PASS (no new issues introduced; targeted findings addressed)
  • sim:nsh (CMake): PASS (build + NSH smoke)
  • qemu-armv7a:nsh (CMake): PASS (build + NSH smoke)

fixed