feat: proper uid/gid + euid/egid implementation with permission enforcement

feat: proper uid/gid + euid/egid implementation with permission enforcement

Kernel:
- struct process: added euid/egid (effective uid/gid) fields
- process_fork_create: now inherits uid/gid/euid/egid from parent
  (previously left at 0 from memset)
- process_clone_create: also inherits euid/egid
- setuid/setgid: permission checks — only euid==0 can set arbitrary
  uid/gid; unprivileged processes can only set to their real uid/gid
- New syscalls: geteuid (88), getegid (89), seteuid (90), setegid (91)
- vfs_check_permission(): checks owner/group/other rwx bits against
  process euid/egid and file uid/gid/mode
- open() now calls vfs_check_permission() for R/W/RW access
- chmod: only root or file owner can change mode
- chown: only root can change ownership
- Added EACCES (13) to errno.h

ulibc:
- Added SYS_GETUID (52), SYS_GETGID (53), SYS_CHMOD (50),
  SYS_CHOWN (51), SYS_GETEUID (88), SYS_GETEGID (89),
  SYS_SETEUID (90), SYS_SETEGID (91)
- Added getuid/getgid/geteuid/getegid/seteuid/setegid wrappers

All 19/19 smoke tests pass.