Add printf style constructor to CPLString

[Lucian Plesea]

CPLString and it's associated Printf method are often chained together to produce a temporary string, to the point that a new function, CPLOPrintf was introduced. This patch provides a CPLString formatting constructor, which also replaces the zero terminated string assignment constructor.

[Even Rouault]

I'm afraid we can't just remove the CPLString( const char *pszStr ) constructor, in case something would rely on copying a string that would contain the % character. Now that could cause segfaults.

char* foo = read from file
CPLString osStr( foo );

[Lucian Plesea]

Ah, yes, didn't think of that one case, I was too happy that I got it working. One could use the append() in that case, but that would be too much.