New utility: gamemodelist (#346)

* New utility: gamemodelist

While trying out gamemode on Ubuntu 18.04 I had trouble figuring out
whether or not my games were running with gamemode enabled.  I wrote this
utility which prints all processes loaded with the gamemode shared
library.

- [x] Added utility to `data/` folder.
- [x] Update meson installer.
- [x] Included section 1 manual.
- [x] Updated README for Ubuntu 18.04 build instructions.  Steam supports
  Ubuntu 18.04.

I'm open to feedback and generally this should work for any distrobution
since it makes use of the Linux `/proc` filesystem.  [Learn more about
`/proc`][1].

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation/filesystems/proc.rst?h=v5.15.12

data/meson.build

@@ -51,6 +51,13 @@ install_data(
     install_mode: 'rwxr-xr-x',
 )
 
+# Install script to find processes with gamemode lib in runtime
+install_data(
+    files('gamemodelist'),
+    install_dir: path_bindir,
+    install_mode: 'rwxr-xr-x',
+)
+
 # Configure and install man pages
 gamemoded_manpage = configure_file(
     input: files('gamemoded.8.in'),
@@ -74,6 +81,17 @@ install_man(
     install_dir: join_paths(path_mandir, 'man1')
 )
 
+gamemodelist_manpage = configure_file(
+    input: files('gamemodelist.1.in'),
+    output: 'gamemodelist.1',
+    configuration: data_conf,
+)
+
+install_man(
+    gamemodelist_manpage,
+    install_dir: join_paths(path_mandir, 'man1')
+)
+
 if with_examples
     example_manpage = configure_file(
         input: files('gamemode-simulate-game.1.in'),