return values of socket functions are ssize_t (not int) in compliance with POSIX

deprecated API (zmq_msg_recv/zmq_msg_send) still returns int
Require pkg-config, use pkg-config to pull seek libsodium.
Merge pull request #1209 from pmienk/master

Simplify libsodium inclusion
Correct declaration of HAVE_LIBSODIUM.
Merge pull request #1210 from pmienk/master

Correct libsodium inclusion.
Remove build and runtime dependencies on e2fsprogs and e2fsprogs-devel.
Fix some typos
Fix builds/msvc/Makefile.am to include all properties files in the build

I attempted to fix up the properties files manifests in the Makefile,
but neglected the end-of-line escape characters.  It was good enough
to pass whatever make dist on Linux did, but I had no Windows build
env to test in.  The broken change was in commit de4a442.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #1212 from dstftw/fix-typos

Fix some typos
Merge pull request #1213 from dmick/master

Fix builds/msvc/Makefile.am to include all properties files in the build
Append to libzmq_la_CPPFLAGS and libzmq_la_LIBADD.
Merge pull request #1207 from hintjens/master

Releasing 4.1.0 rc1 and moving development master to 4.2.0.
Add support for POLLPRI flag.

This commit adds a ZMQ_POLLPRI flag that maps to poll()'s POLLPRI
flag.
This flags does nothing for OMQ sockets. It's only useful for raw
file descriptor (be it socket or file).
This flag does nothing if poll() is not the underlying polling
function. So it is Linux only.
Merge pull request #1216 from xaqq/pollpri

Add support for POLLPRI flag.
Merge pull request #1214 from johntconklin/master

Remove build and runtime dependencies on e2fsprogs and e2fsprogs-devel.
Merge pull request #1217 from johntconklin/master

Append to libzmq_la_CPPFLAGS and libzmq_la_LIBADD.
Support both --with-* and --without-* options for libsodium and pgm
Add support for --with/--without libsodium and --with/--without pgm
command line options.
Update travis script to configure with libsodium.
Merge pull request #1218 from johntconklin/master

Support --with and --without options for libsodium and pgm