GDB/MI: Document support for -exec-run --start in -list-features

GDB/MI: Document support for -exec-run --start in -list-features

This adds "exec-run-start-option" in the output of the -list-features
commands, allowing front-ends to easily determine whether -exec-run
supports the --start option.

gdb/ChangeLog:

        * mi/mi-main.c (mi_cmd_list_features): add "exec-run-start-option".
        * NEWS: Expand the entry documenting the new -exec-run --start
        option to mention the corresponding new entry in the output of
        "-list-features".

gdb/doc/ChangeLog:

	* gdb.texinfo (GDB/MI Miscellaneous Commands): Document the new
	"exec-run-start-option" entry in the output of the "-list-features"
	command.

gdb/testsuite/ChangeLog:

        * gdb.mi/mi-start.exp: Add test verifying that -list-features
        contains "exec-run-start-option".