-Wwrite-strings: execute_command calls with string literals

-Wwrite-strings: execute_command calls with string literals

This is ugly, but it's just making the uglyness explicit.

All these places would better be calling some API function directly
instead of going through execute_command & friends...

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* event-top.c (command_line_handler): Add cast to execute_command
	call.
	* record-btrace.c (cmd_record_btrace_bts_start)
	(cmd_record_btrace_pt_start, cmd_record_btrace_start)
	(cmd_record_btrace_start): Add cast to execute_command call.
	* record-full.c (record_full_goto_insn):
	* record.c (record_start, record_stop): Add cast to
	execute_command_to_string calls.
	(cmd_record_start): Add cast to execute_command calls.