improve fd exhaustion situation about opensmtpd

[poolpOrg]

Diff that fixes fd exhaustion situation in smtp process sent to @ericfaurot and @clongeau
I will work on the control process fd exhaustion situation and it should be committed today ;-)

[poolpOrg]

All uses of env->sc_maxconn and availdesc() need to be put to rest.

We can rely on getdtablesize(), getdtablecount() and whatever fd reserve.

Diff in progress.

[poolpOrg]

A big part of the fd exhaustion handling has been committed, I'll deal with the remaining tommorrow ;-)

[poolpOrg]

fd exhaustion tested on control process and smtp process, everything is fine, closing ticket