init: allow entering of network namespaces

Add the ability to enter a network namespace when launching a service.
Typical usage of this would be something similar to the below:

on fs
  exec ip netns add namespace_name

service vendor_something /vendor/...
  capabilities <lower than root>
  user not_root
  enter_namespace net /mnt/.../namespace_name

Note changes to the `ip` tool are needed to create the namespace in
the correct directory.

Bug: 73334854
Test: auto team verified
Merged-In: Ifa91c873d36d69db399bb9c04ff2362518a0b07d
Change-Id: Ifa91c873d36d69db399bb9c04ff2362518a0b07d
(cherry picked from commit aead51b418e70a49191bc0cc5b67c92c969ae699)
diff --git a/init/service.h b/init/service.h
index bcf1943..87c9ac8 100644
--- a/init/service.h
+++ b/init/service.h
@@ -124,6 +124,9 @@
     using OptionParser = Result<Success> (Service::*)(const std::vector<std::string>& args);
     class OptionParserMap;
 
+    Result<Success> SetUpMountNamespace() const;
+    Result<Success> SetUpPidNamespace() const;
+    Result<Success> EnterNamespaces() const;
     void NotifyStateChange(const std::string& new_state) const;
     void StopOrReset(int how);
     void ZapStdio() const;
@@ -136,6 +139,7 @@
     Result<Success> ParseConsole(const std::vector<std::string>& args);
     Result<Success> ParseCritical(const std::vector<std::string>& args);
     Result<Success> ParseDisabled(const std::vector<std::string>& args);
+    Result<Success> ParseEnterNamespace(const std::vector<std::string>& args);
     Result<Success> ParseGroup(const std::vector<std::string>& args);
     Result<Success> ParsePriority(const std::vector<std::string>& args);
     Result<Success> ParseInterface(const std::vector<std::string>& args);
@@ -179,6 +183,8 @@
     std::vector<gid_t> supp_gids_;
     CapSet capabilities_;
     unsigned namespace_flags_;
+    // Pair of namespace type, path to namespace.
+    std::vector<std::pair<int, std::string>> namespaces_to_enter_;
 
     std::string seclabel_;