FEAT: making UNSET! as a conditionally "TRUTHY"

related issues:
metaeducation/rebol-issues#564
metaeducation/rebol-issues#850

It reverts changes discussed in above issues as I consider this behavior to be more useful and it is compatible with Rebol2 and Red.

My main reason is to be able use PRINT inside ALL blocks without need to worry if these would affect the evaluation.

Like in this case:
```
	if all [
		not headers
		any [
			all [
				d1: find conn/data crlfbin
				d2: find/tail d1 crlf2bin
				print "server using standard content separator of #{0D0A0D0A}"
			]
			all [
				d1: find conn/data #{0A}
				d2: find/tail d1 #{0A0A}
				print "server using malformed line separator of #{0A0A}"
			]
		]
	][ ... ]

```

src/boot/natives.r

@@ -132,7 +132,7 @@ do: native [
 
 either: native [
 	{If TRUE condition return first arg, else second; evaluate blocks by default.}
-	condition
+	condition [any-type!]
 	true-branch
 	false-branch
 	/only "Suppress evaluation of block args."
@@ -188,7 +188,7 @@ halt: native [
 
 if: native [
 	{If TRUE condition, return arg; evaluate blocks by default.}
-	condition
+	condition [any-type!]
 	true-branch
 	/only "Return block arg instead of evaluating it."
 ]
@@ -307,7 +307,7 @@ try: native [
 
 unless: native [
 	{If FALSE condition, return arg; evaluate blocks by default.}
-	condition
+	condition [any-type!]
 	false-branch
 	/only "Return block arg instead of evaluating it."
 ]
@@ -818,7 +818,7 @@ log-e: native [
 
 not: native [
 	{Returns the logic complement.}
-	value {(Only FALSE and NONE return TRUE)}
+	value [any-type!] {(Only FALSE and NONE return TRUE)}
 ]
 
 square-root: native [

src/core/n-control.c

@@ -294,7 +294,7 @@ enum {
 	while (index < SERIES_TAIL(block)) {
 		index = Do_Next(block, index, 0); // stack volatile
 		ds = DS_POP;  // volatile stack reference
-		if (!IS_FALSE(ds) && !IS_UNSET(ds)) return R_TOS1;
+		if (!IS_FALSE(ds)) return R_TOS1;
 	}
 	return R_NONE;
 }

src/mezz/base-defs.r

@@ -136,7 +136,7 @@ scalar?: func [
 
 true?: func [
 	"Returns true if an expression can be used as true."
-	val ; Note: No [any-type!] - we want unset! to fail.
+	val [any-type!] ;- we want unset! not to fail.
 ] [not not :val]
 
 quote: func [

src/tests/run-tests.r3

@@ -18,6 +18,7 @@ wrap load %units/dh-test.r3
 wrap load %units/port-test.r3
 wrap load %units/checksum-test.r3
 wrap load %units/enum-test.r3
+wrap load %units/conditional-test.r3
 
 wrap load %units/crash-test.r3
 

src/tests/units/conditional-test.r3

@@ -0,0 +1,32 @@
+Rebol [
+	Title:   "Rebol conditional test script"
+	Author:  "Olds"
+	File: 	 %conditional-test.r3
+	Tabs:	 4
+	Needs:   [%../quick-test-module.r3]
+]
+
+~~~start-file~~~ "Conditional"
+
+===start-group=== "Dealing with unset! value in conditions"
+	--test-- "any"
+		--assert unset? any [() 2]
+		--assert true?  any [() 2]
+	--test-- "all"
+		--assert true? all []
+		--assert true? all [()]
+		--assert 3 = all [() 1 2 3]
+		--assert unset? all [1 ()]
+		--assert 1 = if all [1 ()][1]
+		--assert 1 = either all [()][1][2]
+	--test-- "any and all"
+		if any [
+			all [false x: 1 ()]
+			all [true  x: 2 ()]
+		][ x: 2 * x]
+		--assert x = 4
+
+
+===end-group===
+
+~~~end-file~~~