[WMS] Changes to help dealing with multiple versions

[dmorissette]

In preparation for some work on the MapContext (OWS Context IE), I need to do a
few changes to make it easier to deal with multiple versions. At the moment we
use string comparisons to compare versions but this is inefficient and leads to
complex and hard to read code.

Instead I'll create some utility functions in mapows.c and macros/constants to
parse a verison string, turn it into an integer, and then do all comparisons as
integers.

[dmorissette]

Working on this now.

[dmorissette]

BTW, I'm planning to do those version handling changes initially in the map
context code, but then also to the WMS and WFS client/server code. So any
comments welcome ASAP.

[dmorissette]

I have committed two new functions in mapows.c: msOWSParseVersionString() and
msOWSGetVersionString(), and a few #defines for the version constants (e.g.
OWS_1_0_0, etc.). 

Also modified mapcontext.c to handle OWS version tests using integer comparisons.

Next steps are WMS and WFS. Adding Steve to CC since I may also have to do the
same changes in the WCS code.

[dmorissette]

I have completed the changes in mapwms.c and mapwmslayer.c as well. BTW, all
this was done in 4.3 CVS only.

I won't touch the WFS and WCS code since they deal only with a single version
for now. If we ever upgrade to support multiple versions of WFS or WCS, then
please have a look at the WMS code and use the same method for testing protocol
versions.

Marking as fixed.