remote: Make vcont_builder a class

remote: Make vcont_builder a class

gdb/ChangeLog:
2018-05-22  Pedro Alves  <palves@redhat.com>

	* remote.c (vcont_builder): Now a class.  Make all data members
	private.
	(vcont_builder) <vcont_builder, restart, flush, push_action>:
	Declare methods.
	(vcont_builder_restart): Rename to ...
	(vcont_builder::restart): ... this.
	(vcont_builder_flush): Rename to ...
	(vcont_builder::flush): ... this.
	(vcont_builder_push_action): Rename to ...
	(vcont_builder::push_action): ... this.
	(remote_target::commit_resume): Adjust.