Grammar corrected/recognize acl flags

I'm an old guy and very familiar with CVS and other legacy SCM tools. This new-fangled GIT world is a steep learning curve for me so I hope I didn't break anything with the fork and pull process after following the Bootcamp/fork-a-repo tutorial.

My little patches to ./lib/libspl/include/sys/mntent.h and ./cmd/mount_zfs/mount_zfs.c just do the following:

1). A minor grammar error was corrected  in main() when the ENOENT case is handled.

2). In both files, the header and the C code, these options are now accepted as passed through by mount in util-linux-2.24 when it execs mount.zfs:

acl
noacl
posixacl