gdb: rename async_init_signals to gdb_init_signals

gdb: rename async_init_signals to gdb_init_signals

The async_init_signals has, for some time, dealt with async and sync
signals, so removing the async prefix makes sense I think.

Additionally, as pointed out by Pedro:

  .....

The comments relating to SIGTRAP and SIGQUIT within this function are
out of date.

The comments for SIGTRAP talk about the signal disposition (SIG_IGN)
being passed to the inferior, meaning the signal disposition being
inherited by GDB's fork children.  However, we now call
restore_original_signals_state prior to forking, so the comment on
SIGTRAP is redundant.

The comments for SIGQUIT are similarly out of date, further, the
comment on SIGQUIT talks about problems with BSD4.3 and vfork,
however, we have not supported BSD4.3 for several years now.

Given the above, it seems that changing the disposition of SIGTRAP is
no longer needed, so I've deleted the signal() call for SIGTRAP.

Finally, the header comment on the function now called
gdb_init_signals was getting quite out of date, so I've updated it
to (hopefully) better reflect reality.

There should be no user visible change after this commit.