added some 'const' annotations

Some API functions were difficulty to use because they require 'char *'
arguments although 'char const *' would suffice.  Patch adds 'const'
annotations to API functions.

Signed-off-by: Enrico Scholz <ensc@ensc.de>
removed xstrdup() from public API

patch renames 'xstrdup()' to '_iniparser_xstrdup()' to avoid conflicts
in projects which are using this very generic name too.

Signed-off-by: Enrico Scholz <ensc@ensc.de>