We currently set attach_flag when attaching to a process, so we should
make sure to unset it when forking a new process. Otherwise attach_flag
would remain set after forking, if the previous process associated with
the inferior was attached to.
gdb/ChangeLog:
* target.c (target_pre_inferior): Unset attach_flag.
gdb/testsuite/ChangeLog:
* gdb.base/run-after-attach.exp: New test file.
* gdb.base/run-after-attach.c: New test file.