Fix style issues in man pages and commands help

* Remove 'zfs snap' from zfs help message (OpenZFS sync)
* Update zfs(8) to suggest 'snap' can be used as an alias for 'snapshot'
* Enforce 80 columns limit in help messages
* Remove zfs_disable_dup_eviction from zfs-module-parameters(5)
* Expose zfs_scan_max_ext_gap as a kernel module parameter.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7087

man/man5/zfs-module-parameters.5

@@ -1253,17 +1253,6 @@ See also \fBzio_dva_throttle_enabled\fR.
 Default value: \fB1000\fR%.
 .RE
 
-.sp
-.ne 2
-.na
-\fBzfs_disable_dup_eviction\fR (int)
-.ad
-.RS 12n
-Disable duplicate buffer eviction
-.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
-.RE
-
 .sp
 .ne 2
 .na

man/man8/zfs.8

@@ -2459,7 +2459,7 @@ The given bookmark is destroyed.
 .Nm
 .Cm snapshot
 .Op Fl r
-.Oo Fl o Ar property Ns = Ns value Oc Ns ...
+.Oo Fl o Ar property Ns = Ns Ar value Oc Ns ...
 .Ar filesystem Ns @ Ns Ar snapname Ns | Ns Ar volume Ns @ Ns Ar snapname Ns ...
 .Xc
 Creates snapshots with the given names.
@@ -2467,6 +2467,9 @@ All previous modifications by successful system calls to the file system are
 part of the snapshots.
 Snapshots are taken atomically, so that all snapshots correspond to the same
 moment in time.
+.Nm zfs Cm snap
+can be used as an alias for
+.Nm zfs Cm snapshot.
 See the
 .Sx Snapshots
 section for details.