Add 'minimal' USE flag. It turns off the build of most clients and all fonts, as...

Add 'minimal' USE flag. It turns off the build of most clients and all fonts, as well as a number of drivers I guessed would be mostly unused by people wanting this functionality (video drivers turned off only for x86). It also stops installing the man pages and docs, as well as Xvfb and Xnest. Finally, it doesn't install Gentoo custom cursors. On a minimal test build with USE="-3dfx +3dnow -bitmap-fonts -cjk -debug -dlloader -dmx -doc -font-server -hardened -insecure-drivers -ipv6 +minimal +mmx -nls -opengl -pam -sdk +sse -static -truetype-fonts -type1-fonts -xprint -xv", the total installation was 28M.