Fix pipe end socket fd left open leak on linux

[martlaak-gmail]

lws_context_destroy() left pipe end fd open, causing fd leak visible in application open file desctiptions list. When application was doing multiple context create/destroys cycles, it was finally causing "too many files open" error in application.

[ERobsham]

FYI, this PR (and issue) is also aimed at plugging the fd leak.