Change strdup to use MsStrdup throughout codebase

[geographika]

See issue #5277 - fixes Windows memory issues in conjunction with using msFree

The new pull request hopefully addresses the comments from @tbonfort in a previous pull request:

#5313 (comment)

• There's still a call to strdup in mapcopy.h
• The calls to strdup in mapscript should also probably be changed
• As you noted, you can remove the mapserver provided strdup() function and the HAVE_STRDUP cmake check

The removal of the replacement strdup function in mapstring.c may mean if any new code doesn't use msStrdup then it will crash on Windows (or at least in debugger mode - strdup still exists even if it is deprecated).

[jmckenna]

@geographika can you also create a pull request for this to branch-7-0 ?