one <= 0 test is an oversight.

Use boilerplate for vmod_debug

We have code which relies on it being able to contain negative
values as error indications, but 'ssize_t' from unistd.h is a bridge
too far for vrt.h which is meant to be _incredibly_ portable to
avoid problems at compile time.

... to bring "more nuance" (quote phk) to the director health
state output, the list callback needs to be able to query
VRT_Health() of the director's backends, so, in turn, we also
need a CTX here.

Ref https://github.com/varnishcache/varnish-cache/pull/2888#issuecomment-460258147

Of the following out-of-tree vmods, none uses the .list callback,
so I expect minimal vmod author uprise:

	- saintmode (varnish-modules)
	- all_healthy
	- dynamic
	- cluster
	- weightadjust (someome(tm) needs to update to vdi_methods)

* move -p documentation from CLI "help backend.list" to sphinx

* Add note on the fact that the health state as seen from VCL may differ

As VRT_Healthy needs a VRT_CTX, we move vcl_get_ctx and vcl_rel_ctx
to cache_varnishd.h scope as VCL_Get_CliCtx and VCL_Rel_CliCtx.

It has been discussed before if all varnishd cli commands should have
a VRT_CTX, so the natural solution would seem  to add one in or close
to VCLS_Poll(), yet CLI commands need specific ctxes (e.g. vcl.load
needs VCL_MET_INIT), so (for now) it appears sensible to limit the
ctx' scope to individual backend method calls.

Fixes #2887

Change name for enums and args to have "enum_" and "arg_" prefixes.

Minimal changes to the vmods to react to these changes.

freely update comments and documentation.

Ref c0903203

my previous commit 7767fc32 was
incaccurate

Ref d8ae26b5

Ref d8ae26b5

Ie:

	vmod.cache_req_body

instead of

	func_cache_req_body

Part of #2863

Closes #2882

acceptable whitespace to SP and TAB.

Part of fix for #2882

In other news:  I hate autocrap

Aka remove linux blinders.

This reverts commit f81b172d.

Refs #2884

This makes it easier to start the UDS-listening peer, and/or set
permissions on the socket file, after starting Varnish or loading
VCL with a UDS backend.

Closes #2884