gdb: rename things related to step over chains

gdb: rename things related to step over chains

Rename step_over_queue_head to global_thread_step_over_chain_head, to
make it more obvious when reading code that we are touching the global
queue.  Rename all functions that operate on it to have "global" in
their name, to make it clear on which chain they operate on.

I also reworded a few comments in gdbthread.h.  They implied that the
step over chain is per-inferior, when in reality there is only one
global chain, as far as I understand.