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/UnixTypes.h

@@ -52,6 +52,7 @@
 #define MS_NOEXEC (1 << 1)
 #define MS_NOSUID (1 << 2)
 #define MS_BIND (1 << 3)
+#define MS_RDONLY (1 << 4)
 
 #define PERF_EVENT_MALLOC 1
 #define PERF_EVENT_FREE 2