Only leave dprintf inserted if it is marked as persistent (PR breakpoints/17012)

Only leave dprintf inserted if it is marked as persistent (PR breakpoints/17012)

On Linux native, if dprintfs are inserted when detaching, they are left
in the inferior which causes it to crash from a SIGTRAP. It also happens
with dprintfs on remote targets, when set disconnected-dprintf is off.

The rationale of the line modified by the patch was to leave dprintfs
inserted in order to support disconnected dprintfs. However, not all
dprintfs are persistent. Also, there's no reason other kinds of
breakpoints can't be persistent either. So this replaces the bp_dprintf
check with a check on whether the location is persistent.

bl->target_info.persist will be 1 only if disconnected-dprintf is on and
we are debugging a remote target. On native, it will always be 0,
regardless of the value of disconnected-dprintf. This makes sense, since
disconnected dprintfs are not supported by the native target.

One issue about the test is that it does not pass when using
--target_board=native-extended-gdbserver, partly due to bug 17302 [1].

One quick hack I tried for this was to add a useless "next" between the
call to getpid() and detach, which avoids the bug. There is still one
case where the test fails, and that is with:

- breakpoint always-inserted on
- dprintf-style agent
- disconnected-dprintf on

What happens is that my detach does not actually detach the process,
because some persistent commands (the disconnected dprintf) is present.
However since gdbserver is ran with --once, when gdb disconnects,
gdbserver goes down and takes with it all the processes it spawned and
that are still under its control (which includes my test process).
When the test checks if the test process is still alive, it obvisouly
fails. Investigating about that led me to ask a question on the ML [2]
about the behavior of detach.

Until the remote case is sorted out, the problematic test is marked as
KFAIL.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=17302
[2] https://sourceware.org/ml/gdb/2014-08/msg00115.html

gdb/Changelog:

	PR breakpoints/17012
	* breakpoint.c (remove_breakpoints_pid): Skip removing
	breakpoint if it is marked as persistent.

gdb/testsuite/ChangeLog:

	PR breakpoints/17012
	* gdb.base/dprintf-detach.c: New file.
	* gdb.base/dprintf-detach.exp: New file.