Make anon clustering more compatible.

[kib]

Summary

Make the clustering enabling knob more fine-grained by providing a setting where the allocation with hint is not clustered. This is aimed to be somewhat more compatible with e.g. go 1.4 which expects that hinted mmap without MAP_FIXED does not change the allocation address.

Now the vm.cluster_anon can be set to 1 to only cluster when no hints, and to 2 to always cluster. Default value is 1.

Requested by: peter

[emaste]

LGTM

[emaste]

Precedent for documenting 0/1/2 options:

kern.geom.eli.visible_passphrase: Visibility of passphrase prompt (0 = invisible, 1 = visible, 2 = asterisk)
kern.vt.splash_cpu_style: Draw logo style (0 = Alternate beastie, 1 = Beastie, 2 = Orb)
vfs.timestamp_precision: File timestamp precision (0: seconds, 1: sec + ns accurate to 1/HZ, 2: sec + ns truncated to us, 3+: sec + ns (max. precision))
vfs.write_behind: Cluster write-behind; 0: disable, 1: enable, 2: backed off
hw.hn.tx_taskq_mode: TX taskqueue modes: 0 - independent, 1 - share global tx taskqs, 2 - share event taskqs
hw.pci.do_power_nodriver: Place a function into D3 state when no driver attaches to it.  0 means disable.  1 means conservatively place devices into D3 state.  2 means aggressively place devices into D3 state.  3 means put absolutely everything in D3 state.
hw.pcic.pd6722_vsense: Select CL-PD6722's VSENSE method.  VSENSE is used to determine the voltage of inserted cards.  The CL-PD6722 has two methods to determine the voltage of the card.  0 means assume a 5.0V card and do not check.  1 means use the same method that the CL-PD6710 uses (default).  2 means use the same method as the CL-PD6729.  2 is documented in the datasheet as being the correct way, but 1 seems to give better results on more laptops.
dev.pcm.#.XXX.vchanmode: vchan format/rate selection: 0=fixed, 1=passthrough, 2=adaptive
dev.uart.0.pps_mode: pulse mode: 0/1/2=disabled/CTS/DCD; add 0x10 to invert, 0x20 for narrow pulse
dev.netmap.admode: Adapter mode. 0 selects the best option available,1 forces native adapter, 2 forces emulated adapter

To me the 0=foo, 1=bar, 2=baz style is easiest to interpret

[kib]

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