Rename "system-preset" source dir to "presets"

I want to add presets/user/ later. This mirrors the layout for units:
we have units/ and units/user. The advantage is that we avoid having yet
another directory at the top level.

meson.build

@@ -2383,6 +2383,7 @@ subdir('units')
 subdir('sysctl.d')
 subdir('sysusers.d')
 subdir('tmpfiles.d')
+subdir('presets')
 subdir('hwdb')
 subdir('network')
 subdir('man')
@@ -2399,8 +2400,6 @@ install_subdir('factory/etc',
 
 install_data('xorg/50-systemd-user.sh',
              install_dir : xinitrcdir)
-install_data('system-preset/90-systemd.preset',
-             install_dir : systempresetdir)
 install_data('modprobe.d/systemd.conf',
              install_dir : modprobedir)
 install_data('README',

presets/meson.build

@@ -0,0 +1,19 @@
+# SPDX-License-Identifier: LGPL-2.1+
+#
+# Copyright 2017 Zbigniew Jędrzejewski-Szmek
+#
+# systemd is free software; you can redistribute it and/or modify it
+# under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation; either version 2.1 of the License, or
+# (at your option) any later version.
+#
+# systemd is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with systemd; If not, see <http://www.gnu.org/licenses/>.
+
+install_data('90-systemd.preset',
+             install_dir : systempresetdir)