support for https domain alias with subdirectories

Added support for https alias with subdirectories.

For example:
https://ssl.com/example.com/wp-admin/ instead of https://example.ssl.com/wp-admin/

Will be activated with set HTTPS_DOMAIN_ALIAS_SUBDOMAIN to false.
HTTPS_DOMAIN_ALIAS will be used as the subdirectory in this case.

I also added a missing hook for mulisite support and a new define to make the rewrite of attachement urls optional.