awk: Flag -Ft as deprecated behavior

Upstream is poised to deprecate the -Ft wart in one true awk. None of
the other awks do this, and the gawk maintainer says that he's had no
requests for it in gawk in 30 years maintaining it. github can find a
few instances of it in the wild. As such, warn that it's deprecated and
will go away in the future.

MFC After:		3 days
Sponsored by:		Netflix

contrib/one-true-awk/main.c

@@ -89,8 +89,10 @@ static const char *
 setfs(char *p)
 {
 	/* wart: t=>\t */
-	if (p[0] == 't' && p[1] == '\0')
+	if (p[0] == 't' && p[1] == '\0') {
+		WARNING("-Ft to imply tab separator is deprecated behavior.");
 		return "\t";
+	}
 	return p;
 }
 

usr.bin/awk/awk.1

@@ -23,7 +23,7 @@
 .\" THIS SOFTWARE.
 .\"
 .\"	$FreeBSD$
-.Dd $Mdocdate: June 6 2020 $
+.Dd July 30, 2021
 .Dt AWK 1
 .Os
 .Sh NAME
@@ -812,3 +812,11 @@ to it.
 .Pp
 The scope rules for variables in functions are a botch;
 the syntax is worse.
+.Sh DEPRECATED BEHAVIOR
+One True Awk has accpeted
+.Fl Ft
+to mean the same as
+.Fl F\t
+to make it easier to specify tabs as the separator character.
+Upstream One True Awk has deprecated this wart in the name of better
+compatibility with other awk implementations like gawk and mawk.