"source", foreground execution commands, and target-async

Sourcing a GDB script that runs foreground execution commands in
succession fails if the target can async:

 Breakpoint 1, main () at ../../../src/gdb/testsuite/gdb.base/source-execution.c:36
 36        func1 ();
 (gdb) source ../../../src/gdb/testsuite/gdb.base/source-execution.gdb
 ../../../src/gdb/testsuite/gdb.base/source-execution.gdb:21: Error in sourced command file:
 Cannot execute this command while the selected thread is running.
 (gdb) FAIL: gdb.base/source-execution.exp: source source-execution.gdb

That is, after a foreground execution command, GDB moves on to the
following command immediately before waiting for the previous command
to complete.

https://sourceware.org/ml/gdb-patches/2011-09/msg00037.html (b4a14fd0)
addressed this for command lists, Python's gdb.execute, etc., but
missed "source".  Fixed now in the same way.

gdb/
2014-03-25  Pedro Alves  <palves@redhat.com>

	* cli/cli-script.c (script_from_file): Force the interpreter to
	sync mode.

gdb/testsuite/
2014-03-25  Pedro Alves  <palves@redhat.com>
	    Doug Evans  <dje@google.com>

	* gdb.base/source-execution.c: New file.
	* gdb.base/source-execution.exp: New file.
	* gdb.base/source-execution.gdb: New file.