Make NVIDIA shared memory (shmem) feature optional

adding conditional compilation guards around shmem-related headers,
making the shmem feature optional in build configurations, and
changing the default value from enabled to disabled.

Fixes JIRA https://jirasw.nvidia.com/browse/DGXOPENBMC-18776

Change-Id: Ic6ac5005c2bfe32885132a9d5f85677f62520910

meson_options.txt

diff --git a/meson_options.txt b/meson_options.txt
index 671e8cc..289b6fc 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -9,7 +9,7 @@
     'shmem',
     type: 'feature',
     description: 'Use NVIDIA Shared-Memory IPC',
-    value: 'disabled',
+    value: 'enabled',
 )
 option(
     'reset-metrics',