Make remote follow fork 'Detaching' message match native

Make remote follow fork 'Detaching' message match native

This patch fixes a couple of failures in gdb.base/foll-vfork.exp for
extended-remote targets.  The failures were the result of the
verbose/debug "Detaching..." messages in infrun.c:follow_fork_inferior
not matching what was expected in the extended-remote case.

The path modifies the ptids used in the messages to ensure that they
print "process nnn" instead of (possibly) "Thread nnn.nnn".  The
detach is a process-wide operation, so we need to use a process-
style ptid regardless of what type of ptid target_pid_to_str returns.

Tested on x86_64 GNU/Linux, native, remote, extended-remote.

gdb/

	* infrun.c (follow_fork_inferior): Ensure the use of
	process-style ptids (pid,0,0) in verbose/debug "Detaching"
	messages.