meson: remove deprecated get_pkgconfig_variable

Since meson 0.56, the `get_pkgconfig_variable` has been deprecated.  In
meson 0.58 the `get_variable` was enhanced to no longer require the
`pkgconfig` keyword argument.  Ensure meson 0.58 is required and update
the usage of all `get_pkgconfig_variable` and `get_variable` to be the
modern variant.

Change-Id: Icfc9a4ff0929cfe33ff5773988016e5ee39337ac
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index a60e22b..039f39a 100644
--- a/meson.build
+++ b/meson.build
@@ -1,6 +1,6 @@
 project('phosphor-user-manager',
         'cpp',
-        version: '0.1', meson_version: '>=0.57.0',
+        version: '0.1', meson_version: '>=0.58.0',
         default_options: [
           'warning_level=3',
           'werror=true',
@@ -119,7 +119,7 @@
 
 
 systemd_system_unit_dir = systemd_dep.get_variable(
-    pkgconfig: 'systemdsystemunitdir'
+    'systemdsystemunitdir'
 )
 
 install_data(