(extremely minor QETH cosmetic changes)
QETH DBGTRC tweaks:

1. Delete DBGTRC, rename DBGTRC2 -> DBGTRC: devnum always displayed.
2. Adjust many DBGTRC statements as appropriate.
3. Add many more DBGTRC stmts to identify every control req/rsp.
QETH .h header tweaks + new sig2str() function:

1. New OSA_TIMEOUTUS macro constant
2. Rename UCLEVEL -> QUCLEVEL
3. Rename MAC_PROMISC -> MAC_TYPE_PROMISC
4. Move QDSIG_xxx constants from header to source file ahead of sig2str() function.
More reliable QETH shutdown handling
QETH init handler netmask/prefix consistency fix.
Fix QETH/OSA tuntap interface handling:

1. Only create tuntap interface ONCE.
2. Do NOT enable tuntap interface when it's created.
3. Enable/disable tuntap interface only when guest requests it.
4. Where/when tuntap interface is enabled/disabled (i.e. which guest control request triggers the disable/enable) depends on current layer mode (L2 vs. L3).

z/Linux layer 2 now works again and z/OS layer 3 now works MUCH more reliably as well.