Split non-target-dependent code out of target_attach routines.

Split non-target-dependent code out of target_attach routines.

* target.h:  Comments on target_attach args and results.
* infcmd.c (attach_command): Check for existing execution, call
target_attach, set up terminal status and wait_for_inferior, wait
for the attach status, and do normal_stop.
* inftarg.c (child_attach):  Remove target independent stuff.
* remote-adapt.c (adapt_attach):  Ditto.
* remote-mm.c (mm_attach):  Ditto.
* remote-udi.c (udi_attach):  Ditto.
* remote-vx.c (vx_attach):  Ditto.

Cleanup.

* remote-hms.c (hms_attach):  Remove completely, it was useless.
* remote-mm.c, remote-hms.c, remote-udi.c, remote-adapt.c:  Remove
commented-out start_remote calls.
* remote-hms.c, remote-adapt.c, remote-mm.c, remote-udi.c: Remove
DENTER and DEXIT macros and their calls.  Use a real debugger --
like gdb -- to see what functions are being called when.