Kernel: Support read-only filesystem mounts

This adds support for MS_RDONLY, a mount flag that tells the kernel to disallow
any attempts to write to the newly mounted filesystem. As this flag is
per-mount, and different mounts of the same filesystems (such as in case of bind
mounts) can have different mutability settings, you have to go though a custody
to find out if the filesystem is mounted read-only, instead of just asking the
filesystem itself whether it's inherently read-only.

This also adds a lot of checks we were previously missing; and moves some of
them to happen after more specific checks (such as regular permission checks).

One outstanding hole in this system is sys$mprotect(PROT_WRITE), as there's no
way we can know if the original file description this region has been mounted
from had been opened through a readonly mount point. Currently, we always allow
such sys$mprotect() calls to succeed, which effectively allows anyone to
circumvent the effect of MS_RDONLY. We should solve this one way or another.

Kernel/FileSystem/Custody.h

@@ -54,6 +54,7 @@ public:
     String absolute_path() const;
 
     int mount_flags() const { return m_mount_flags; }
+    bool is_readonly() const;
 
 private:
     Custody(Custody* parent, const StringView& name, Inode&, int mount_flags);