pathconf: Define a new name called _PC_CASE_INSENSITIVE

Some FreeBSD file systems (msdosfs and optionally ZFS)
handle case insensitive file name lookup.
NFS (v3 and 4) provide an attribute to indicate this
capability, but without this patch, the NFS server does
not know if the file system does support this.

This patch adds the pathconf name_PC_CASE_INSENSITIVE
that can be used by the NFS server or user programs to
check for this feature.

Reviewed by:	kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D53349

sys/fs/msdosfs/msdosfs_vnops.c

@@ -1944,6 +1944,9 @@ msdosfs_pathconf(struct vop_pathconf_args *ap)
 	case _PC_HAS_HIDDENSYSTEM:
 		*ap->a_retval = 1;
 		return (0);
+	case _PC_CASE_INSENSITIVE:
+		*ap->a_retval = 1;
+		return (0);
 	default:
 		return (vop_stdpathconf(ap));
 	}

sys/kern/vfs_default.c

@@ -458,6 +458,7 @@ vop_stdpathconf(struct vop_pathconf_args *ap)
 		case _PC_HAS_NAMEDATTR:
 		case _PC_HAS_HIDDENSYSTEM:
 		case _PC_CLONE_BLKSIZE:
+		case _PC_CASE_INSENSITIVE:
 			*ap->a_retval = 0;
 			return (0);
 		default:

sys/sys/unistd.h

@@ -160,6 +160,7 @@
 #define	_PC_XATTR_EXISTS	_PC_HAS_NAMEDATTR	/* Solaris Compatible */
 #define	_PC_HAS_HIDDENSYSTEM	68
 #define	_PC_CLONE_BLKSIZE	69
+#define	_PC_CASE_INSENSITIVE	70
 #endif
 
 /* From OpenSolaris, used by SEEK_DATA/SEEK_HOLE. */