nshlib: Add stderr redirection support

[fangpeina]

Summary

This PR implements stderr redirection support in NSH (NuttShell), enabling Bash-like syntax for redirecting standard error output. This enhancement improves NSH compatibility with standard Unix shells and provides better error handling capabilities.

Features Added:

• Support 2> operator for stderr redirection (overwrite mode)
• Support 2>> operator for stderr redirection (append mode)
• Support 2>&1 syntax to redirect stderr to stdout
• Works with both foreground and background commands

Usage Examples:

nsh> ls noexists 2> err.log           # Redirect stderr to file
nsh> ls noexists 2> err.log &         # Background with stderr redirect
nsh> ls noexists 2>> err.log          # Append stderr to file
nsh> sh < /dev/ttyS0 > /dev/ttyS0 2> /dev/ttyS1 &    # Multiple redirections
nsh> sh < /dev/ttyS0 > /dev/ttyS0 2>&1 &             # Merge stderr to stdout

Impact

• API Changes: Modified nsh_vtbl_s structure to include stderr file descriptor (internal API only, no user-facing changes)
• User Experience: Users can now redirect error messages separately from standard output, improving script debugging and error logging
• Security: No security implications
• Compatibility: Fully backward compatible - existing NSH commands and scripts continue to work unchanged

Testing

Test Configuration:

• Platform: Simulation (sim:nsh)
• Config: sim:nsh

Runtime Test Results:

Test 1: Basic stderr redirection (overwrite mode)

nsh> ll 2> err.log
nsh> cat err.log
nsh: ll: command not found

Test 2: Stderr append mode

nsh> ll 2> err.log
nsh> cat err.log
nsh: ll: command not found
nsh> ls noexists 2>> err.log
nsh> cat err.log
nsh: ll: command not found
nsh: ls: stat failed: 2

Test 3: Redirect stderr to stdout (2>&1)

nsh> ls > log 2>&1
nsh> cat log
/:
 bin/
 data/
 dev/
 err.log
 etc/
 log
 proc/
 tmp/

[fangpeina]

merged