Command abbreviation in define

When defining a new macro, "command" is not recognized as an alias for
"commands":

 (gdb) define breakmain
 Type commands for definition of "breakmain".
 End with a line saying just "end".
 >break main
 >command
 >echo "IN MAIN\n"
 >end
 (gdb)

There is a special case for while-stepping, where 'ws' and 'stepping' are
recognized explicitely. Instead of adding more special cases, this change
uses cli-decode.

gdb/ChangeLog:
	* cli/cli-decode.c (find_command_name_length): Make it extern.
	* cli/cli-decode.h (find_command_name_length): Declare.
	* cli/cli-script.c (command_name_equals, line_first_arg):
	New functions.
	(process_next_line): Use cli-decode to parse command names.
	(build_command_line): Make args a constant pointer.

gdb/testsuite/ChangeLog:

	* gdb.base/define.exp: Add test for command abbreviations
	in define.

gdb/cli/cli-decode.c

@@ -1255,7 +1255,9 @@ find_cmd (const char *command, int len, struct cmd_list_element *clist,
   return found;
 }
 
-static int
+/* Return the length of command name in TEXT.  */
+
+int
 find_command_name_length (const char *text)
 {
   const char *p = text;