nshlib: add du command support

[Junbo-Zheng]

Summary

Add a du (disk usage) command to NSH that recursively summarizes the size of each path argument in 1K-blocks, or in human-readable form with -h. Also supports -s (summary only), -a (all files), -d N (max-depth), and -h, matching GNU du semantics.

Impact

• Users: New du command in NSH; no existing command or behavior
  changed.
• Build: Adds CONFIG_NSH_DISABLE_DU (default follows
  DEFAULT_SMALL) to omit the command on small builds.
• Hardware: None -- NSH addition, board-independent.
• Documentation: None -- usage shown via NSH help.

Testing

Built and run on sim:nsh (host Ubuntu 22.04):

cmake -B out/nuttx_sim_nsh -S nuttx -DBOARD_CONFIG=sim:nsh -GNinja
ninja -C out/nuttx_sim_nsh
./out/nuttx_sim_nsh/nuttx

-h prints one decimal via integer arithmetic, float printf:

nsh> du
397449  /data/test/elf
71993   /data/test/coredump
5       /data/test/log2
3251    /data/test/log1
472700  /data/test

nsh> du -h
388.1M  /data/test/elf
70.3M   /data/test/coredump
4.1K    /data/test/log2
3.1M    /data/test/log1
461.6M  /data/test

Host Ubuntu 22.04 du on the same directory

$ du
397456  ./elf
72000   ./coredump
8       ./log2
3252    ./log1
472720  .

$ du -h
389M  ./elf
71M   ./coredump
8.0K  ./log2
3.2M  ./log1
462M  .

[acassis]

@Junbo-Zheng please fix:

FAILED: nuttx 
: && /tools/ccache/bin/arm-none-eabi-gcc  --specs=nosys.specs   -Wl,--print-memory-usage -Wl,--entry=__start -nostdlib -Wl,--gc-sections -Wl,--cref -Wl,-Map=nuttx.map @CMakeFiles/nuttx.rsp -o nuttx  && :
/tools/gcc-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/13.2.1/../../../../arm-none-eabi/bin/ld: apps/libapps.a(nsh_fscmds.c.o): in function `du_recursive':
/github/workspace/sources/apps/nshlib/nsh_fscmds.c:1960:(.text.du_recursive+0xae): undefined reference to `nsh_getdirpath'
Memory region         Used Size  Region Size  %age Used