Gentoo Portage. What exactly ‘revdep-rebuild’ command does

The state in the last paragraph is sufficient - add /media, add the group vboxsf and reboot (which I didn't when I tried this before).

Using portage you can do this with package.env. The right place to look for the documentation is http://dev.gentoo.org/~zmedico/portage/doc/portage.html#config-bashrc-ebuild-phase-hooks. Basically the way you use it is as follows. First you create (assuming standard setup without custom ROOT) a file in /etc/portage/env. For example, you can create a file /etc/portage/env/paxmark

#paxmark

#Set this to what you need it to be
PAXFLAGS="-p"

post_src_install() {
  find "${INSTALL}" -type x -print0 |xargs -0 -n 1 paxctl $PAXFLAGS
}

Then for all packages you want this to apply to you add an entry to /etc/portage/package.env :

#package.env example for paxmark
sys-apps/gcc paxmark

This will apply the paxmark script to the package that has it specified. Alternatively you can also create a /etc/portage/bashrc script for global overrides (be very careful with that).

A general warning though, as you can add pre and post hooks to all phases this can be dangerous. Be careful with what you do as all your packages that use the hook have now become no more robust than your hook script. (The above example for pax marking should be fine).