add SELinux policy module

Closes #46

Fixes #45

Fixes #44

Fixes #33

The configuration file will never be overwritten automatically.
Therefore, changes need to be applied by the user manually.

Use char instead of char* for the single character and don't do this
weird casting thing where a char gets stored in the pointer.

Read config directly from environment variables

This reverts commit cdb223542a.

Network is not need anymore to start postsrsd.

Remove addrinfo flags

Remove flags since they require the network to be up and routable for no reason.

Closes #14
Closes #17

As reported in #25

Suggested by maxfieb (#21)

Fixes #21

The Solaris libc has some issues with alternating input and output
on sockets passed to fdopen(). Apparently there is a shared buffer
which has to be emptied, either by fseek() or fflush(), whenever
the direction of the data flow changes.

Fixes #13

A couple of small fixes: closing listen sockets in child and ignoring EINTR errors from main poll