init: Add gentle_kill service property

If a service specifies gentle_kill, attempt to stop it will send SIGTERM
instead of SIGKILL. After 200ms, it will issue a SIGKILL.

Bug: 249043036
Test: atest CtsInitTestCases:init#GentleKill
      Added in next patch
Change-Id: Ieb0e4e24d31780aca1cf291f9d21d49cee181cf2
diff --git a/init/service_parser.h b/init/service_parser.h
index 54503dd..670a5c6 100644
--- a/init/service_parser.h
+++ b/init/service_parser.h
@@ -53,6 +53,7 @@
     Result<void> ParseDisabled(std::vector<std::string>&& args);
     Result<void> ParseEnterNamespace(std::vector<std::string>&& args);
     Result<void> ParseGroup(std::vector<std::string>&& args);
+    Result<void> ParseGentleKill(std::vector<std::string>&& args);
     Result<void> ParsePriority(std::vector<std::string>&& args);
     Result<void> ParseInterface(std::vector<std::string>&& args);
     Result<void> ParseIoprio(std::vector<std::string>&& args);