LibCore+Everywhere: Remove ArgsParser::add*(char const*&)

This is not guaranteed to always work correctly as ArgsParser deals in
StringViews and might have a non-properly-null-terminated string as a
value. As a bonus, using StringView (and DeprecatedString where
necessary) leads to nicer looking code too :^)

Userland/Utilities/pkill.cpp

@@ -24,7 +24,7 @@ ErrorOr<int> serenity_main(Main::Arguments args)
 
     bool case_insensitive = false;
     bool echo = false;
-    char const* pattern = nullptr;
+    StringView pattern;
     int signal = SIGTERM;
 
     Core::ArgsParser args_parser;