nshlib: hard/soft credential drop and sudo via PR_NSHCRED_AUTHSETEID

[Abhishekmishra2808]

Summary

• Hard-drop non-root NSH logins with setres*(uid,uid,uid); soft-drop from root-originated sessions keeps saved-root.
• Wire su / sudo to prctl(PR_NSHCRED_AUTHSETEID) for password verify + euid/egid raise; enable the agent with PR_NSHCRED_ENABLE at NSH init.
• Improve useradd UID allocation (passwd_append next UID ≥ 1000) and expand ostest multiuser coverage for AUTHSETEID / hard-drop paths.
• Depends on the companion nuttx PR sched: supplementary groups, setresuid/setresgid, and PR_NSHCRED_AUTHSETEID nuttx#19767

Impact

NSH sessions can drop to a real non-root identity and escalate only after an in-kernel passwd check, which is what makes su/sudo usable with hard-dropped logins. Builds without CONFIG_SCHED_USER_IDENTITY are unchanged.

Testing

nsh# useradd testuser UserPass1!
nsh# cat /tmp/passwd
root:$pbkdf2-sha256$10000$uwK3GNfobksYkqa5H9-Rcw$kNsxMZi26PeLxdWKgFjNvZpOoouP_DcHyO2mwyACVc0:0:0:/
testuser:$pbkdf2-sha256$10000$LCsDACzp0RA26xvaHPsY2g$gfCJXfLINNOKNIpVqepoUaYJiMbB8ojezAlosZJnrXY:1000:1000:/
nsh# su testuser
nsh$ id
uid=1000(testuser) suid=0(root) gid=1000 sgid=0(root) groups=1000
nsh$ su root
password: 
nsh# id
uid=0(root) gid=0(root) groups=0(root)
nsh#