Merge pull request #2 from davidcorry/master

Add iOS
Add inventories for most Snakebite platforms:

* AIX 7.1 (gcc, xlc)
* HPUX 11.31 (gcc)
* NetBSD 5.1 (gcc)
* Solaris 9 (gcc), 10 (gcc, suncc), 11 (gcc, suncc)
* Tru64 Unix (DEC CC)
First cut of generated header-availability table.
Sort columns in decreasing order of number of headers available (within categories).
Improve formatting. Distinguish compiler-dependent headers.
Move all data files into a subdirectory.
Pull all of the embedded data out of scansys.py.

* sorthdr.py and setcommon.py merged into scansys.py
* prerequisites now defined in an .ini file
* compute the headers-to-probe list from data/b-*, only
* remove data/r-*
* add some entries to data/b-ucom so that the probe set is unchanged
* scansys.py now does proper command line parsing; can be told to look
  for data/ and prereqs.ini elsewhere
Make Python scripts executable.
Merge prep side A: match filename changes from side B
Merge prep side B: match move to subdir from side A
Merge tblgen work from last week
License tagging; move web resources to subdirectory.
Make scansys.py backward compatible all the way to Python 2.0.
Make scansys.py backward compatible all the way to Python 2.0.
argh
Document --prereqs and --debug in scansys.py lead-in comments.
Fix a handful of glitches and infelicitous names in scansys.py.

You know you were hacking too late last night when you get up the next
morning and discover you committed an incomplete merge.
Refactor scansys.py with an engine class.

The functions that do all the real work no longer have five or more state
arguments threaded through them.
Add compiler-identification annotation to scansys output.
Document scansys's absolute pathname requirement.

Also document scansys's need to work with py2.0 in README.md, and shuffle
the code around a little so all the compatibility kludges are at the top.