meson: use install_symlink() where applicable

Now our baseline of meson is 0.62, hence install_symlink() can be used.

Note, install_symlink() implies install_emptydir() for specified
install_dir. Hence, this also drops several unnecessary
install_emptydir() calls.

Note, the function currently does not support 'relative' and 'force' flags,
so several 'ln -frsT' inline calls cannot be replaced.

src/run/meson.build

@@ -8,11 +8,9 @@ executables += [
         },
 ]
 
-install_emptydir(bindir)
-
-meson.add_install_script(sh, '-c',
-                             ln_s.format(bindir / 'systemd-run',
-                                         bindir / 'run0'))
+install_symlink('run0',
+                pointing_to : 'systemd-run',
+                install_dir : bindir)
 
 custom_target(
         'systemd-run0',