man: Define doc-operating-system-NetBSD string variables

This way we do not depend on the installed groff being new enough.
Signed-off-by: Guillem Jover <guillem@hadrons.org>

man/errc.3bsd

@@ -145,6 +145,7 @@ if ((fd = open(block_device, O_RDONLY, 0)) == -1)
 .Xr printf 3 ,
 .Xr strerror 3
 .Sh HISTORY
+.ds doc-operating-system-NetBSD-7.0 7.0
 The functions
 .Fn errc ,
 .Fn verrc ,

man/strtoi.3bsd

@@ -226,6 +226,7 @@ function is a
 .Nx
 extension.
 .Sh HISTORY
+.ds doc-operating-system-NetBSD-7.0 7.0
 The
 .Fn strtoi
 function first appeared in

man/strtonum.3bsd

@@ -145,6 +145,7 @@ is an
 .Ox
 extension.
 .Sh HISTORY
+.ds doc-operating-system-NetBSD-8.0 8.0
 The
 .Fn strtonum
 function first appeared in

man/strtou.3bsd

@@ -220,6 +220,7 @@ the range given was invalid, i.e.
 .Xr strtoull 3 ,
 .Xr strtoumax 3
 .Sh STANDARDS
+.ds doc-operating-system-NetBSD-7.0 7.0
 The
 .Fn strtou
 function is a