core: rename EXEC_APPLY_PERMISSIONS → EXEC_APPLY_SANDBOXING

"Permissions" was a bit of a misnomer, as it suggests that UNIX file
permission bits are adjusted, which aren't really changed here. Instead,
this is about UNIX credentials such as users or groups, as well as
namespacing, hence let's use a more generic term here, without any
misleading reference to UNIX file permissions: "sandboxing", which shall
refer to all kinds of sandboxing technologies, including UID/GID
dropping, selinux relabelling, namespacing, seccomp, and so on.

src/core/execute.h

@@ -255,7 +255,7 @@ static inline bool exec_context_restrict_namespaces_set(const ExecContext *c) {
 }
 
 typedef enum ExecFlags {
-        EXEC_APPLY_PERMISSIONS = 1U << 0,
+        EXEC_APPLY_SANDBOXING  = 1U << 0,
         EXEC_APPLY_CHROOT      = 1U << 1,
         EXEC_APPLY_TTY_STDIN   = 1U << 2,
         EXEC_NEW_KEYRING       = 1U << 3,