Remove kill_inferior_fast, in favor of target_kill, which goes

Remove kill_inferior_fast, in favor of target_kill, which goes
through the target vector.

* inferior.h (kill_inferior_fast): remove declaration.

* main.c (disconnect): call quit_cover using catch_errors rather
than calling kill_inferior_fast directly.  New way goes through
the target vector, handles attached processes, and writes
command history if appropriate.
(quit_cover): new function, wrapper for quit_command.

* convex-xdep.c, go32-xdep.c, hppabsd-xdep.c, hppahpux-xdep.c,
infptrace.c, procfs.c:  Removed all instances of kill_inferior_fast,
inlining them into the local kill_inferior when needed.