idmap mount support (#1523)

* Add idmap mount support

* Further replacement of std::filesystem::path to fs::path

* Update docs regarding idmap support

mkdocs/docs/config/kernel-permissions-check.md

@@ -16,4 +16,4 @@ help. Like [export-support](export-support.md) this is mostly for
 debugging.
 
 This option is a kernel mount option so unable to be changed at
-runtime.
+runtime. It controls the `default_permissions` FUSE kernel option.

mkdocs/docs/config/options.md

@@ -86,6 +86,9 @@ config file.
 * **[export-support](export-support.md)=BOOL**: Sets a low-level FUSE
   feature intended to indicate the filesystem can support being
   exported via NFS. (default: true)
+* **[kernel-permissions-check](kernel-permissions-check.md)=BOOL**:
+  Controls the FUSE `default_permissions` option. Primarily for
+  debugging. (default: true)
 * **security_capability=BOOL**: If false return ENOATTR when xattr
   security.capability is queried. (default: true)
 * **[xattr](xattr.md)=passthrough|noattr|nosys**: Runtime control of

mkdocs/docs/faq/technical_behavior_and_limitations.md

@@ -201,3 +201,15 @@ contention and therefore lower performance than Linux.
 Additionally, mergerfs [utilizes a cache for supplemental
 groups](../known_issues_bugs.md#supplemental-user-groups) due the the
 high cost of querying that information.
+
+
+## Does mergerfs support idmap?
+
+Yes. At least in so far as it's been enabled now the FUSE itself
+allows a filesystem to indicate it is allowed.
+
+Requires that
+[kernel-permissions-check](../config/kernel-permissions-check.md) be
+enabled (the default.)
+
+If there are any usage issues contact the [author](../support.md).