Allow tunefs to include '_' as a legal character in label names

to make it consistent with newfs. Document the legality of '_'
in label names in both tunefs(8) and newfs(8).

PR:           235182
Submitted by: darius@dons.net.au
Reviewed by:  Conrad Meyer
MFC after:    3 days
Sponsored by: Netflix

sbin/newfs/newfs.8

@@ -89,6 +89,7 @@ See
 for details.
 .It Fl L Ar volname
 Add a volume label to the new file system.
+Legal characters are alphanumerics and underscores.
 .It Fl N
 Cause the file system parameters to be printed out
 without really creating the file system.

sbin/tunefs/tunefs.8

@@ -112,6 +112,7 @@ By default
 sets it to half of the space reserved to minfree.
 .It Fl L Ar volname
 Add/modify an optional file system volume label.
+Legal characters are alphanumerics and underscores.
 .It Fl l Cm enable | disable
 Turn on/off MAC multilabel flag.
 .It Fl m Ar minfree

sbin/tunefs/tunefs.c

@@ -189,7 +189,7 @@ main(int argc, char *argv[])
 			name = "volume label";
 			Lvalue = optarg;
 			i = -1;
-			while (isalnum(Lvalue[++i]));
+			while (isalnum(Lvalue[++i]) || Lvalue[i] == '_');
 			if (Lvalue[i] != '\0') {
 				errx(10,
 				"bad %s. Valid characters are alphanumerics.",