filesystems: add group for "application" file system

This group shall cover file systems whose job is to make stuff that
isn't actually storing anything in itself, and isn't just an API file
system, but allows exposing stuff with special semantics in the VFS.

src/basic/filesystems.c

@@ -59,6 +59,14 @@ const FilesystemSet filesystem_sets[_FILESYSTEM_SET_MAX] = {
                 "pipefs\0"
                 "sockfs\0"
         },
+        [FILESYSTEM_SET_APPLICATION] = {
+                .name = "@application",
+                .help = "Application virtual filesystems",
+                .value =
+                "autofs\0"
+                "fuse\0"
+                "overlay\0"
+        },
         [FILESYSTEM_SET_AUXILIARY_API] = {
                 .name = "@auxiliary-api",
                 .help = "Auxiliary filesystem API",

src/basic/filesystems.h

@@ -11,6 +11,7 @@ typedef enum FilesystemGroups {
         /* Please leave BASIC_API first and KNOWN last, but sort the rest alphabetically */
         FILESYSTEM_SET_BASIC_API,
         FILESYSTEM_SET_ANONYMOUS,
+        FILESYSTEM_SET_APPLICATION,
         FILESYSTEM_SET_AUXILIARY_API,
         FILESYSTEM_SET_COMMON_BLOCK,
         FILESYSTEM_SET_HISTORICAL_BLOCK,