[ssl] use DH only if openssl supports it (fixes #2479)

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@2884 152afb58-edef-0310-8abb-c4023f1b3aa9
[network] use constants available at compile time for maximum number of chunks for writev instead of calling sysconf (fixes #2470)

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@2885 152afb58-edef-0310-8abb-c4023f1b3aa9