check SO_ERROR after nonblocking connect

[psumbera]

On Solaris, several memcached_errors tests failed because the client treated a nonblocking connect as successful as soon as poll/select reported the socket writable, then immediately attempted a send without first checking SO_ERROR. When the connect had actually failed, send() returned EPIPE, which libmemcached reported as MEMCACHED_SYSTEM_ERROR instead of MEMCACHED_CONNECTION_FAILURE. This change fixes the connect race by checking SO_ERROR in IO_POLL_CONNECT before declaring success, and also normalizes mid-connection write failures by mapping EPIPE/ENOTCONN/ECONNRESET to MEMCACHED_CONNECTION_FAILURE (marking the instance dead).

[psumbera]

This is how test failed on Solaris:

/scratch/psumbera/userland-libmemcached/components/libmemcached/build/i86: ctest -R bin/memcat  -VV
UpdateCTestConfiguration  from :/scratch/psumbera/userland-libmemcached/components/libmemcached/build/i86/DartConfiguration.tcl
Parse Config file:/scratch/psumbera/userland-libmemcached/components/libmemcached/build/i86/DartConfiguration.tcl
UpdateCTestConfiguration  from :/scratch/psumbera/userland-libmemcached/components/libmemcached/build/i86/DartConfiguration.tcl
Parse Config file:/scratch/psumbera/userland-libmemcached/components/libmemcached/build/i86/DartConfiguration.tcl
Test project /scratch/psumbera/userland-libmemcached/components/libmemcached/build/i86
Constructing a list of tests
Done constructing a list of tests
Updating test list for fixtures
Added 0 tests to meet fixture requirements
Checking test dependency graph...
Checking test dependency graph end
test 2
    Start 2: bin/memcat

2: Test command: /scratch/psumbera/userland-libmemcached/components/libmemcached/build/i86/test/runtests "bin/memcat"
2: Working Directory: /scratch/psumbera/userland-libmemcached/components/libmemcached/build/i86/test
2: Test timeout computed to be: 1500
2: Filters: bin/memcat
2: 
2: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2: runtests is a Catch v2.13.6 host application.
2: Run with -? for options
2: 
2: -------------------------------------------------------------------------------
2: bin/memcat
2:   connection failure
2: -------------------------------------------------------------------------------
2: ./libmemcached-1.1.4/test/tests/bin/memcat.cpp:18
2: ...............................................................................
2:
2: ./libmemcached-1.1.4/test/tests/bin/memcat.cpp:21: FAILED:
2:   REQUIRE_THAT( output, Contains("CONNECTION FAILURE") )
2: with expansion:
2:   "Fatal error for key 'memcat': (fa6a7028) SYSTEM ERROR(Broken pipe),  host:
2:   localhost:13146 -> ./libmemcached-1.1.4/src/libmemcached/io.cc:346
2:   " contains: "CONNECTION FAILURE"
2:
2: ===============================================================================
2: test cases:  1 |  0 passed | 1 failed
2: assertions: 40 | 39 passed | 1 failed
2:
1/1 Test #2: bin/memcat .......................***Failed    5.23 sec

0% tests passed, 1 tests failed out of 1

Total Test time (real) =   5.24 sec

The following tests FAILED:
          2 - bin/memcat (Failed)

Errors while running CTest