Change all instances of strdup to msStrdup

[geographika]

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

I left the following in mapstring.c alone. I presume this shouldn't be called anyway if only msStrdup is used?

#ifndef HAVE_STRDUP
char  *strdup(char *s)
{
  char  *s1;
  ...

[tbonfort]

• 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

[geographika]

Superseded by #5321