Fix PR breakpoints/16101: gdb.base/dprintf.exp agent-printf failures with non-Z0-supporting gdbservers

After a previous patch that was committed by Pedro (0000e5cc), trying
to set a dprintf with with a GDBserver that doesn't support agent
commands at all now throws an error.  But the dprintf tests still fail
with some GDBserver targets because they doesn't try to handle the
case of the server reporting support for breakpoint commands, but not
be able to use those in combination with Z0 (because Z0 isn't actually
supported, for example):

 FAIL: gdb.base/dprintf.exp: 1st dprintf, agent
 FAIL: gdb.base/dprintf.exp: 2nd dprintf, agent
 FAIL: gdb.base/dprintf.exp: dprintf info 2 (pattern 4)

Similarly for the MI test.

This patch makes the tests handle this scenario.

Tested with native, and native gdbserver on x86_64 Fedora 17.

Also tested with the native-gdbserver.exp board hacked with:

 set GDBFLAGS "${GDBFLAGS} -ex \"set remote breakpoint-commands off\""

(actually, "set remote breakpoint-commands off" is presently broken,
so this was on top of a fix for that command.)

which results in:
 (gdb) PASS: gdb.base/dprintf.exp: 2nd dprintf, gdb
 set dprintf-style agent
 warning: Target cannot run dprintf commands, falling back to GDB printf
 warning: Target cannot run dprintf commands, falling back to GDB printf
 (gdb) UNSUPPORTED: gdb.base/dprintf.exp: set dprintf style to agent

gdb.sum:
 Running target native-gdbserver
 Running ../../../src/gdb/testsuite/gdb.base/dprintf.exp ...
 PASS: gdb.base/dprintf.exp: dprintf
 PASS: gdb.base/dprintf.exp: dprintf foo
 PASS: gdb.base/dprintf.exp: dprintf 29
 PASS: gdb.base/dprintf.exp: dprintf foo,"At foo entry\n"
 PASS: gdb.base/dprintf.exp: ignore $bpnum 1
 PASS: gdb.base/dprintf.exp: dprintf 26,"arg=%d, g=%d\n", arg, g
 PASS: gdb.base/dprintf.exp: dprintf info 1
 PASS: gdb.base/dprintf.exp: break 27
 PASS: gdb.base/dprintf.exp: 1st dprintf, gdb
 PASS: gdb.base/dprintf.exp: 2nd dprintf, gdb
 UNSUPPORTED: gdb.base/dprintf.exp: set dprintf style to agent
 PASS: gdb.base/dprintf.exp: Set dprintf style to an unrecognized type

And also with the native-gdbserver.exp board hacked with:

 set GDBFLAGS "${GDBFLAGS} -ex \"set remote Z-packet off\""

which results in:
 (gdb) continue
 Continuing.
 Warning:
 Cannot insert breakpoint 3: Target doesn't support breakpoints that have target side commands.
 Cannot insert breakpoint 4: Target doesn't support breakpoints that have target side commands.

 (gdb) UNSUPPORTED: gdb.base/dprintf.exp: 1st dprintf, agent

gdb.sum:
 Running target native-gdbserver
 Running ../../../src/gdb/testsuite/gdb.base/dprintf.exp ...
 PASS: gdb.base/dprintf.exp: dprintf
 PASS: gdb.base/dprintf.exp: dprintf foo
 PASS: gdb.base/dprintf.exp: dprintf 29
 PASS: gdb.base/dprintf.exp: dprintf foo,"At foo entry\n"
 PASS: gdb.base/dprintf.exp: ignore $bpnum 1
 PASS: gdb.base/dprintf.exp: dprintf 26,"arg=%d, g=%d\n", arg, g
 PASS: gdb.base/dprintf.exp: dprintf info 1
 PASS: gdb.base/dprintf.exp: break 27
 PASS: gdb.base/dprintf.exp: 1st dprintf, gdb
 PASS: gdb.base/dprintf.exp: 2nd dprintf, gdb
 PASS: gdb.base/dprintf.exp: set dprintf style to agent
 UNSUPPORTED: gdb.base/dprintf.exp: 1st dprintf, agent
 PASS: gdb.base/dprintf.exp: Set dprintf style to an unrecognized type

(One of the new comments mentions breakpoint always-inserted mode.
Actually testing with breakpoint always-inserted mode fails these
dprintf tests, due to the way they are written.  But that'll take a
more substancial rewrite of the tests, so I'm leaving that for another
day.)

gdb/testsuite/
2014-03-24  Hui Zhu  <hui@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	PR breakpoints/16101
	* gdb.base/dprintf.exp: Use unsupported rather than changing the
	test pass/fail messages.  Detect missing support for dprintf when
	breakpoints are actually inserted.
	* gdb.base/mi-dprintf.exp: Detect missing support for dprintf when
	breakpoints are actually inserted.
	* lib/mi-support.exp (mi_run_cmd_full): Return -1 if continue
	fails.