Split the network-only and compression-only parts of buffers into
the appropriate modules.

We need to use lib/fs/winlib.h here so that we can use
GetTickCount64.

I would love to declare that XP is dead, and everybody has
GetTickCount64.

Note that procmon does *not* go here, since procmon needs to
integrate with the event loop.

Note that the workqueue code does *not* go here: it is logically at
a higher level, since it needs to use libevent and the networking
stack.

This code was in compat_threads, since it was _used_ for efficiently
notifying the main libevent thread from another thread.  But in
spite of its usage, it's fundamentally a part of the network code.

Also, move "unescape_string()" to encoding too, since it's about
encoding data as C strings.

The "conffile" module knows about includes and filesystem access,
whereas confline doesn't.  This will make it possible to put these
functions into libraries without introducing a cycle.

libtor-encoding is about various ways to transform data to and from
character sequences.

Also move our ntohll/htonll functions.