Add some OSX-specific files to .gitignore

_Noreturn kw must appear before the function name as per C11 grammar and
clang7 is the first one to enforce that.

Ecb_noreturn macro is defined using _Noreturn keyword if C11 support is
detected; otherwise __noreturn__ gcc attribute is used. The later must
appear after the function parameters list.  These two methods to
annotate a function are inherently incompatible in terms of the token
location relative to the function declaration.

Use gcc attribute exclusively since it is always available.

Explicitly close net.box connection on error.

Partially reverts squashed 316d4e3a

This regression introducing during replacing port_ffi with port_buf
by 2fcce449

Linux-specific option obsolete since kernel 2.2