gdb/linux-nat: factor ptrace-detach code to new detach_one_pid function

The following patch will add some code paths that need to ptrace-detach
a given PID.  Factor out the code that does this and put it in its own
function, so that it can be re-used.

Change-Id: Ie65ca0d89893b41aea0a23d9fc6ffbed042a9705