util: Introduce buffer size for carrying /proc/self/fd/N path

There's ... a number of places where we want to do something
with /proc/self/fd/%d path. Each time we guess buffer size
that is enough for this. Make standard constant for this and
save some space on stack and drop args for some functions.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files.c

@@ -940,7 +940,7 @@ out:
 
 static int fchroot(int fd)
 {
-	char fd_path[32];
+	char fd_path[PSFDS];
 
 	/*
 	 * There's no such thing in syscalls. We can emulate