Merge pull request #779 from sappo/master

Problem: zpubsub has been removed thus the examples shall be removed as well
Problem: Android ndk #defines errno as a preprocessor macro

Solution: Guard errno declaration in zsys
Android macro defined here: https://android.googlesource.com/platform/prebuilts/ndk/+/master/9/platforms/android-19/arch-arm/usr/include/errno.h
Merge pull request #780 from jemc/errno_guard

Problem: Android ndk #defines errno as a preprocessor macro
Simply configure.ac to prevent contradictory discovery of libzmq.
Problem: Android needs to provide special LDFLAGS to libtool linking
Solution: Accommodate the LIBTOOL_EXTRA_LDFLAGS env var
Merge pull request #781 from pmienk/master

Simply configure.ac to prevent contradictory discovery of libzmq.
Merge pull request #782 from jemc/master

Problem: Android needs to provide special LDFLAGS to libtool linking
Problem: misuse of include files in zsys

All system specific include files go into czmq_prelude.h. zsys was using
errno.h for no good reason.

Solution: just remove this, errno is already included in czmq_prelude.h.

Secondary issues fixed on the way:

- zsys was sending its UDP sendto() failures to stderr, I changed this
  to send to zsys_error() as for all other error reports.

- zarmour was including zarmour.h which is redundant -- removed.
Problem: zbeacon contains bogus comments

These were left over from the original work, serve no purpose.

Solution: remove the junk comments.
Problem: zsocket tried to unbind wildcard endpoint

This was not documented and recently changed in ZeroMQ, so now causes
an assertion failure.

Solution: do not use '*' interface in tests, use 127.0.0.1 instead.
Merge pull request #783 from hintjens/master

Problem: misuse of include files in zsys
Update CONTRIBUTING.md
Problem: if WiFi interface changes, zbeacon gets confused

Solution: if sendto() hits an error, this is most likely an ENETUNREACH
error, and we can deal with this by closing the UDP socket and opening
a new one, on whatever interface is available. If there's none then the
zbeacon class loops until there's one available.
Problem: beacon reset did not work properly

Solution: handle sendto return code accurately. Also only show the
"cannot find interface" error once at start of app, rather than at
every poll interval if/when the interface is down.
Problem: PKG_CHECK_MODULES doesn't add to LIBS on android
Solution: Add -lzmq to LIBS manually
Merge pull request #785 from jemc/master

Problem: PKG_CHECK_MODULES doesn't add to LIBS on android
Added base16 support to zarmour
Merge pull request #786 from lovmoen/master

Problem: zarmour does not support base16
Merge pull request #784 from hintjens/master

Problem: zbeacon does not recover from network changes
Problem: Can't install with out of tree build

Solution: Make was searching for czmq.7 file in srcdir while in fact it
is created during compilation, so removing the srcdir reference to let
it proceed.