Factor out "Detaching from program" message printing

Several targets have a copy of the same code that prints

 "Detaching from program ..."

in their target_detach implementation.  Factor that out to a common
function.

(For now, I left the couple targets that print this a bit differently
alone.  Maybe this could be further pulled out into infcmd.c.  If we
did that, and those targets want to continue printing differently,
this new function could be converted to a target method.)

gdb/ChangeLog:
2016-07-01  Pedro Alves  <palves@redhat.com>

	* darwin-nat.c (darwin_detach): Use target_announce_detach.
	* inf-ptrace.c (inf_ptrace_detach): Likewise.
	* nto-procfs.c (procfs_detach): Likewise.
	* remote.c (remote_detach_1): Likewise.
	* target.c (target_announce_detach): New function.
	* target.h (target_announce_detach): New declaration.