Install the nethack util binaries and their documentation.

[heiner]

Using those, we can define our own custom levels, e.g.

1. Use the tar-like tool dlb to extract nhdat.
2. Add a mylevel.des file as per 5. Creating a new level in
   https://nethackwiki.com/wiki/Beginner%27s_guide_to_NetHack_sources
   (the .des files are part of the NLE installation, but not in nhdat as they are sources).
3. Compile that file using lev_comp, producing mylevel.lev.
4. Add the new level to dungeon.def (also part of the NLE installation).
5. Run makedefs -e and dgn_comp dungeon.pdf to produce dungeon.
6. Re-assemble the new set of files using dlb which includes dungeon and mylevel.des.

Example

From patch_nhdat.sh in this commit:

HACKDIR=$(python -c 'import pkg_resources; print(pkg_resources.resource_filename("nle", "nethackdir"), end="")')

TMPDIR=/tmp/patch_nhdat
mkdir $TMPDIR

mkdir $TMPDIR/dat
cd $TMPDIR/dat
cp $HACKDIR/dat/dungeon.def .
patch --ignore-whitespace <<'EOF'
--- dungeon.def      2019-03-01 15:21:08.000000000 +0100
+++ dungeon.def 2020-09-23 19:17:51.000000000 +0200
@@ -15,6 +15,7 @@
 #

 DUNGEON:       "The Dungeons of Doom" "D" (25, 5)
+LEVEL:         "mylevel" "none" @ (1,1)
 ALIGNMENT:     unaligned
 BRANCH:                "The Gnomish Mines" @ (2, 3)
 LEVEL:         "rogue" "R" @ (15, 4)

EOF
$HACKDIR/makedefs -e  # Looks for ../dat/dungeon.def.
$HACKDIR/dgn_comp dungeon.pdf

cp $HACKDIR/dat/oracle.des mylevel.des
patch --ignore-whitespace <<'EOF'
--- mylevel.des       2019-03-01 15:21:08.000000000 +0100
+++ mylevel.des     2020-09-23 19:01:56.000000000 +0200
@@ -5,7 +5,7 @@
 # Oracle level
 #

-LEVEL: "oracle"
+LEVEL: "mylevel"

 ROOM: "ordinary" , lit, (3,3), (center,center), (11,9) {
   OBJECT:('`',"statue"),(0,0),montype:'C',1
EOF
$HACKDIR/lev_comp mylevel.des

mkdir $TMPDIR/contents
cd $TMPDIR/contents
$HACKDIR/dlb xf $HACKDIR/nhdat
cp -f ../dat/dungeon ../dat/mylevel.lev .
$HACKDIR/dlb cf nhdat *

# Now copy nhdat somewhere to use it.

[rockt]

Exciting! LGTM.