LibCore: Add File::read_link() :^)

This is a convenient wrapper around readlink() that hides away the details
of buffers and buffer sizes, and simply returns a String. The best part is it
doesn't rely on PATH_MAX :D

It comes in two versions, for Serenity, where we can pass non-null-terminated
strings to syscalls, and where sys$readlink() returns the total link size, and
for other systems, where we have to copy out the string, and always have to do
two syscalls.

Libraries/LibCore/File.h

@@ -47,6 +47,7 @@ public:
 
     static bool exists(const String& filename);
     static String real_path_for(const String& filename);
+    static String read_link(const StringView& link_path);
 
     virtual bool open(IODevice::OpenMode) override;