Mark all file descriptors involved in raw ssh traffic as binary/raw

GCC v4.7.0, as found in Fedora 17 and newer, fails to build mosh on
ARM because of a narrowing warning, which is then promoted to an error
by -Werror.  This error message is new, and did not occur under
previous versions of GCC.

This patch corrects this error by explicitly casting to the desired
datatype the return value of the offending code, the result of a macro
from glibc to implement htobe16().

This reverts commit 682bbdfe.

AF42 has both the properties of CS42 and the IMM bit.

Whether this has the advantages or disadvantages of just one or
the other, I don't know. With most mac80211 stacks this will
force mosh packets in the VI queue on wireless.

As for pfifo_fast, have to look it up...

Closes #248.

This reverts commit b0e05776.

This reverts commit 63459ed1.

Closes #265.

For pselect() we want to mess with the signal mask, and this is closer to our
intent anyway.

Now that we have select.cc.

It's going to manipulate process-global signal state, so multiple
instances do not make sense.

Closes #235.