Arbitrary mlock() half-memory limit.

[Mark J Roberts]

	/* we may lock at most half of physical memory... */
	/* (this check is pretty bogus, but doesn't hurt) */
	if (locked > num_physpages/2)
		goto out;

I've been running fluidsynth (a synthesizer program) with 700-900MB
instrument sample files on a box with 1GB of memory. It tries to
lock the samples into memory and fails.

This isn't a problem for me, since I don't have swap configured and
the sample data is anonymous-backed, but it's a case in which the
arbitrary limit is clearly pernicious.