plan9port: modify the builder to use the INSTALL script.

Plan9port ships with an INSTALL script. This commit modifies the
builder to use the script instead of a custom build script. The
commit also adds a patch to build fontsrv, which is otherwise
omitted from the build.

Like the way upstream plan9port installs itself, this package also
installs under ~/.nix-profile/plan9 and one has to add the following
to the shell startup scripts to use it.
PLAN9=~/.nix-profile/plan9 export PLAN9
PATH=$PATH:$PLAN9/bin export PATH