Rework desktop and linuxemu bits for c6.

Now that ports defaults to CentOS 6.6, redo the relevant handbook bits,
replacing Fedora 10 with CentOS 6.6. While there, improve the wording a little.

Differential Revision: https://reviews.freebsd.org/D1117
Approved by: wblock, bcr
Approved by: swills (mentor)