functions of sent mail box

function list:

1. read sent mails from (e.g.) `mail/S/SYSOP/send_mail/.DIR`
2. read content of mails (see detail in `bbsmailcon.c`)
3. edit content of mails (see bbseditmail.c for detail)
4. [Most IMPORTANT] forward mail (see bbsfwdmail.c)
5. [?] delete is problematic and lack of test (see bbsdelmail.c)

I did simple test on local and I request a test on devlop-site.
Let user save his/her mail

Change includes:

1. show "备份" when sending mail (actually it had been existed, but been commented. I changed the UI, but still not perfect.)

2. add new function called `post_mail_to_sent_box` in `nju09/BBSLIB.c` which can create file and .DIR in `mail/%c/%s/send_mail/`

3. save, aka backup, in bbssndmail.c
Just merge with IB's modified

[Warning] I can not test after change the path of sent mail.