added pkgconfig for UNIX

1. Added nanomsg.pc file generation for pkg-config (for UNIX only)
2. Added information to README to choose different installation directory

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Missing pkgconfig.in added

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
New test added to tests/tcp.c

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Rigorous validation of TCP connection strings

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Minor renaming (list->item)

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
API for transport-specific socket options added

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Implementation of transport-specific option in the core

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>