fd: move relative path string to library

Use a single static variable located in the library, instead of
having a separate one for each compilation unit.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/fd.h

@@ -57,7 +57,6 @@ namespace crucible {
 
         typedef ResourceHandle<int, IOHandle> Fd;
 
-	static string __relative_path;
 	void set_relative_path(string path);
 	string relative_path();
 

lib/fd.cc

@@ -529,6 +529,8 @@ namespace crucible {
 		THROW_ERROR(runtime_error, "readlink: maximum buffer size exceeded");
 	}
 
+	static string __relative_path;
+
 	string
 	relative_path()
 	{