Make linux_nat_detach/thread_db_detach use the inferior parameter
    
    This patch makes these two functions actually use the inferior parameter
    added by the previous patch, instead of reading inferior_ptid.  I chose
    these two, because they are the one actually used when I detach on my
    GNU/Linux system, so they were easy to test.
    
    I took the opportunity to pass the inferior being detached to
    inf_ptrace_detach_success, so it could use it too.  From there, it made
    sense to add an overload of detach_inferior that takes the inferior
    directly rather than the pid, to avoid having to pass inf->pid only for
    the callee to look up the inferior structure by pid.
    
    gdb/ChangeLog:
    
    	* inf-ptrace.c (inf_ptrace_detach): Adjust call to
    	inf_ptrace_detach_success.
    	(inf_ptrace_detach_success): Add inferior parameter, use it
    	instead of inferior_ptid, pass it to detach_inferior.
    	* inf-ptrace.h (inf_ptrace_detach_success): Add inferior
    	parameter.
    	* inferior.c (detach_inferior): Add overload that takes an
    	inferior object.
    	* inferior.h (detach_inferior): Likewise.
    	* linux-nat.c (linux_nat_detach): Use the inf parameter, don't
    	use inferior_ptid, adjust call to inf_ptrace_detach_success.
    	* linux-thread-db.c (thread_db_detach): Use inf parameter.