Enable extra compiler warnings and fix them.
Don't clobber "unused" registers for non Unix sockets.

On RHEL4.7 x86_64, it seems the 6th register is used by
socketcall(SYS_CONNECT, ...).  This can be reproduced with the
following command:

    proot -B -r centos-5-x86 telnet ...

Before:

    sysenter start: 102, 0x3, 0xffffd140, 0xf7fef530, 0x56572dc0, 0xf7ff6e30, 0xffffd12c) = 0xffffffda [0xffffd12c]
    sysenter end: 102, 0x3, 0xffffd140, 0xf7fef530, 0x56572dc0, 0x56572de0, 0x10) = 0xffffffda [0xffffd12c]
    sysexit start: 102, 0x3, 0xffffd140, 0xf7fef530, 0x56572dc0, 0x56572de0, 0x10) = 0xfffffff2 [0xffffd12c]
    sysexit end: 102, 0x3, 0xffffd140, 0xf7fef530, 0x56572dc0, 0xf7ff6e30, 0xffffd12c) = 0xfffffff2 [0xffffd12c]

Now:

    sysenter start: 102, 0x3, 0xffffd140, 0xf7fef530, 0x56572dc0, 0xf7ff6e30, 0xffffd12c) = 0xffffffda [0xffffd12c]
    sysenter end: 102, 0x3, 0xffffd140, 0xf7fef530, 0x56572dc0, 0xf7ff6e30, 0xffffd12c) = 0xffffffda [0xffffd12c]
    sysexit start: 102, 0x3, 0xffffd140, 0xf7fef530, 0x56572dc0, 0xf7ff6e30, 0xffffd12c) = 0x0 [0xffffd12c]
    sysexit end: 102, 0x3, 0xffffd140, 0xf7fef530, 0x56572dc0, 0xf7ff6e30, 0xffffd12c) = 0x0 [0xffffd12c]
Make the "kompat" extension (-k option) work again.
Fix defects reported by Coverity Prevent.
Make glue files not accessible only if they don't belong to the glue fs.

This commit avoids spurious "can't delete /tmp/proot-$PID-XXXX" messages.
Prepare release v2.4.1.