Enable use of LOG directive in MapScript

[dmorissette]

In the current version of MapServer, the LOG directive is handled only inside
the mapserv CGI and has no effect in MapScript.  We have lots of code generating
msDebug() output to stderr when debug is enabled, but that requires a debug
build and is a pain to use in some circumstances.

I would like to change things so that msDebug() output is set to the file
specified by WEB/LOG if it is set, and to stderr if WEB/LOG is not set. Plus all
msSetError() calls should be logged to that same logfile in MapScript just like
with the mapserv CGI.

Any comments/objections before I make that change? (Other than saying that we
should have done that a few years ago!)