Fix __unix_fcntl

[jserv]

F_SETFL was broken: used the translated command constant instead of the caller's flags from arg, and passed &linuxflags (a pointer) where fcntl expects an int.

This also fixes several related issues in the same function:

• F_GETFL: errno set unconditionally, leaking stale values on success
• F_GETLK: input lock region not populated before kernel query
• F_DUPFD: returned 0 instead of the new file descriptor
• cmd table indexed without bounds check (OOB on bad input)
• unmapped table entries silently mapped to F_DUPFD (index 0)
• F_SETLKW mapped in table but missing from switch (process abort)
• F_GETFD/F_SETFD mapped in table but missing from switch
• default case called err() killing the process instead of EINVAL