gdb/testsuite: Add a way to send multiple init commands

Right now we provide a board info entry, `gdb_init_command', that allows
one to send a single command to GDB before the program to be debugged is
started.  This is useful e.g. for slow remote targets to change the
default "remotetimeout" setting.  Occasionally I found a need to send
multiple commands instead, however this cannot be achieved with
`gdb_init_command'.

This change therefore extends the mechanism by adding a TCL list of GDB
commands to send, via a board info entry called `gdb_init_commands'.
There is no limit as to the number of commands put there.  The old
`gdb_init_command' mechanism remains supported for compatibility with
existing people's environments.

	* lib/gdb-utils.exp: New file.
	* lib/gdb.exp (gdb_run_cmd): Call gdb_init_commands, replacing
	inline `gdb_init_command' processing.
	(gdb_start_cmd): Likewise.
	* lib/mi-support.exp (mi_run_cmd): Likewise.
	* README: Document `gdb_init_command' and `gdb_init_commands'.