use more conservative 30 second default timeout for LINGER

Regression in #1248 can cause lost messages at exit.

Problem: return code of sodium_init() is not checked.

There are two todo comments in curve_client.cpp and curve_server.cpp that suggest
checking the return code of sodium_init() call. sodium_init() returns -1 on error,
0 on success and 1 if it has been called before and is already initalized:
https://github.com/jedisct1/libsodium/blob/master/src/libsodium/sodium/core.c

Use consistent spelling, fix typos.

behavior -> behaviour
initialize -> initialse
initialization -> initialisation

Problem: default LINGER value is insane

No error-checking of setsockopt ZMQ_CURVE_* z85 keys. Solves #1094.

Solution: use a sane value, e.g. 2 seconds

Fixes #1247

Documentation fix for zmq_ctx_term

Solution: document this with a clear warning. It would be
nicer perhaps to change the default LINGER to e.g. a few
seconds. However this could break existing applications.

Fix typo: addres -> address

Problem: qt-android build doesn't fully work for downstream builds

Solution: Fix the qt-android build

Restore structure to targets lost during nonrecursive refactor.

Problem: No builtin way to build for qt-android

Solution: Don't print tar extraction verbosely

Solution: Add qt-android build system and add to travis-ci

Disable IPC testing on ipc-incapable systems in test_term_endpoint.cpp

Added test and updated documentation for unbind wild-card * binded socke...

Unbind socket with real endpoint when binding by wild-card * address

Support both --enable-eventfd and --disable-eventfd options.

Pass --with/--without-libgssapi_krb5 to configure.

Standardize optional libgssapi_krb5 dependency