Document zfs_flags module parameter

Add a table describing the debugging flags that can be set in the zfs_flags
module parameter.  Also change the module_param type to 'uint' so users aren't
shown a negative value. The updated man page text is reproduced below for
convenience.

```
zfs_flags (int)

Set additional debugging flags. The following flags may be
bitwise-or'd together. The ZFS_DEBUG_DBUF_VERIFY and ZFS_DEBUG_DNODE flags are
ignored  unless  ZFS  was  built  with  debugging enabled.

Value   Symbolic Name              Description
    1   ZFS_DEBUG_DPRINTF          Enable dprintf entries in the debug log
    2   ZFS_DEBUG_DBUF_VERIFY      Enable extra dbuf verifications
    4   ZFS_DEBUG_DNODE_VERIFY     Enable extra dnode verifications
    8   ZFS_DEBUG_SNAPNAMES        Enable snapshot name verification
   16   ZFS_DEBUG_MODIFY           Check for illegally modified ARC buffers
   32   ZFS_DEBUG_SPA              Enable spa_dbgmsg entries in the debug log
   64   ZFS_DEBUG_ZIO_FREE         Enable verification of block frees
  128   ZFS_DEBUG_HISTOGRAM_VERIFY Enable extra spacemap histogram verifications

Default value: 0.
```
Signed-off-by: Ned Bass <bass6@llnl.gov>