www/mod_http2-devel: Update to version 1.5.2

• Update to version 1.5.2
• Add mod_proxy_http2 Apache module
• Add missing USE_OPENSSL
• Define PORTDOCS
• Replace pkg-plist with PLIST_FILES

Reviewed by: feld (mentor)
Approved by: feld (mentor)
Differential Revision: D6010