Use std::vector in linux_xfer_osdata_processgroups

This simplifies the code quite a bit, by removing the array of PID_T
that's actually an array of pairs of PID_T.

This code is only used to implement "info os procgroups".  I tested by hand
as well as by running gdb.base/info-os.exp for unix, native-gdbserver
and native-extended-gdbserver.

gdb/ChangeLog:

	* nat/linux-osdata.c: Include algorithm.
	(compare_processes): Remove.
	(struct pid_pgid_entry): New struct.
	(linux_xfer_osdata_processgroups): Use std::vector instead of
	XNEWVEC.