zonectl: display conventional zones better during RZ

[asomers]

Summary

zonectl's Report Zones subcommand displays a tabular list of zones. A
conventional zone's WP column is displayed as 0xffffffffffffffff , the
literal value that the HDD reports. But that's too wide for the column,
causing the text to be misaligned. It's also not really meaningful,
because the Write Pointer isn't really defined for a Conventional zone. Change it to "N/A" to fix the text misalignment.

MFC after: 2 weeks
Sponsored by: ConnectWise

[asomers]

Before:

55880 zones, Maximum LBA 0xda47ffff (3662151679)
Zone lengths are the same, types vary
  Start LBA  Length       WP LBA      Zone Type      Condition      Sequential             Reset
          0,  65536, 0xffffffffffffffff,  Conventional,           NWP,     Sequential,  No Reset Needed
    0x10000,  65536, 0xffffffffffffffff,  Conventional,           NWP,     Sequential,  No Reset Needed

After:

55880 zones, Maximum LBA 0xda47ffff (3662151679)
Zone lengths are the same, types vary
  Start LBA  Length       WP LBA      Zone Type      Condition      Sequential             Reset
          0,  65536,         N/A,  Conventional,           NWP,     Sequential,  No Reset Needed
    0x10000,  65536,         N/A,  Conventional,           NWP,     Sequential,  No Reset Needed

[fuz]

Can we change it to say -1 so it still parses as a number? n/a looks like something scripts will dislike.

[asomers]

In D57512#1318003, @fuz wrote:

Can we change it to say -1 so it still parses as a number? n/a looks like something scripts will dislike.

Yes we can. In fact, I tried that first, but switched to "N/A" because I thought that -1 might look like a mistake. @ken what do you think?