xfsprogs: fixup generate_obfuscated_name()

This series has expanded some since I first posted a version of it back in October.

My original purpose was to fix a bug in the function used by xfs_metadump to obfuscate file and extended attribute names. Arkadiusz Miśkiewicz found out that when he attempted to perform a metadump on a filesystem which had some files whose names used 8-bit characters, it never completed. The cause was a flaw in the algorithm used to obfuscate names which caused an infinite loop when certain names got obfuscated.

I have fixed that bug (in patch 4 of this series), but while I was at it I rearranged the code a bit and did some other cleanups. The first 7 patches in the series are really pretty directly related to that issue.

But while in there I noticed some other somewhat subtle issues and have addressed those in this series as well:
- There is a table that tracks names to ensure duplicates aren't produced as a result of obfuscation. There was a case where a failure to update the table was silently ignored.
- There was still a chance that pathological cases could end up repeating due to finding duplicates. I changed it so it would give up after a fixed number of attempts.
- Obfuscated names had been chosen from an alphabet of all allowable characters. This was more than needed, so I made it create names that were (mostly) printables.
- It was previously possible for an obfuscated name to end up being a duplicate of a real name, because non-obfuscated names were not placed in the name table.

And finally I did a little tidying of the
code that manages the name table.

Below is a summary of the patches, in order.

-Alex

xfsprogs: some things aren't all that special
Some simple code refactoring to make the purpose
of one help function more focused.
xfsprogs: simplify leading '/' handling in generate_obfuscated_name()
Another cleanup, to simplify some other changes to come.
xfsprogs: drop unneeded use of a random character
This tweaks the obfuscated filename algorithm slightly to simplify it, while preserving its original properties.
xfsprogs: adjust rather than start over when invalid byte found
This contains the real fix to the original problem. This patch contains a very long description to explain what's happening. But basically it changes the algorithm so that when a bad (random) character is produced, we don't start over, but instead modify the character (by flipping a bit), resulting in a good character that will be used.
xfsprogs: eliminate a pointless loop in generate_obfuscated_name()
The previous change made a loop in the code no longer needed.
xfsprogs: Replace repeated blocks with a loop
The code is now structured so all of the last 5 characters in an obfuscated name are handled identically. This just takes advantage of that by using a loop over them.
xfsprogs: use pointers in generate_obfuscated_name()
Sort of a cleanup, replacing array references with pointers.
xfsprogs: ensure dup table always has entry for obfuscated name
This just makes sure we create an entry in the name table before we pass back an obfuscated name.
xfsprogs: don't loop on too many dups
This ensures we don't loop forever (or even for very long) if we repeatedly come up with duplicate entries when obfuscating names.
xfsprogs: use printable characters for obfuscated filenames
This restricts the characters used for all but the last five letters of obfuscated file names to just a set of printables.
xfsprogs: fix up the nametable code in db/metadump.c
This encapsulates some code that works with the
name table used in tracking duplicates.
xfsprogs: Drop a typedef in db/metadump.c
This just gets rid of an unnecessary typedef.

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