xl2tpd-control: fix xl2tpd hanged up in "fopen"

[wendy2001011]

This is a fix for xl2tpd hanged up in "fopen" result fifo while working on xl2tpd with OpenWrt.

Root cause is as followings,

xl2tpd-control open result fifo ##fifo readers=1
xl2tpd-control read result fifo
xl2tpd-control close result fifo ##fifo readers=0
xl2tpd fopen result fifo ##xl2tpd is hanged up here to wait readers
xl2tpd-control unlink result fifo
The fix replaces the order of "unlink" and "close" when cleaning up to avoid hang up issue in fopen, and add the retry waiting when reading result fifo to wait xl2tpd fopen result fifo.

A refactoring proposal
There is sequence problem using fifo mechanism between xl2tpd and xl2tpd-control.
To solve the problem entirely, It is better to use socket communication between xl2tpd and xl2tpd-control.
This topic had talked with yousong.
But using socket to refactor this is a big change, I hope to get your suggestions.

What do you think about the refactoring proposal?
Thanks.

[shussain]

Change is in devel branch.

As for the refactoring idea, it seems like a good idea.