Make gdb.base/dprintf.exp use gdb_test_stdio

Make gdb.base/dprintf.exp use gdb_test_stdio

This one needed a larger revamp.  The issue is that the "info
breakpoints" test at the bottom of the file is broken on targets that
can do both server-side dprintf, and inferior I/O, because then
neither the breakpoint numbers match nor the "already hit N times"
output.

Address that by making the test restart gdb from scratch when
switching between dprintf styles.  Test groups are factored into
procedures, and we now use with_test_prefix.  While we're changing
test messages, lowercase a few test messages, and then while at it,
modernize a couple things here and there.

gdb/testsuite/ChangeLog:
2015-07-29  Pedro Alves  <palves@redhat.com>

	* gdb.base/dprintf.exp: Use standard_testfile.  Change
	prepare_for_testing call.
	(srcfile): Don't set.
	(restart): New procedure.
	(test_dprintf): New procecure, use to continue over dprintfs.
	(test_call, test_agent): New procedures, tests moved here.
	Restart gdb and recreate dprintfs.  Adjust expected output.