Revert I/O retry on EINTR patch

[thibaultcha]

This is one of 4 PRs to properly handle EINTR errors in resty-cli (and/or OpenResty if desired by the user).

• (this PR) Revert the "retry on EINTR" patch from bugfix: resume read system call upon EINTR during file readall. #16
• feature: introduced the 'lua_sa_restart' directive, which sets the SA_RESTART flag on nginx workers signal dispositions. lua-nginx-module#1296 Implement an FFI API to enable SA_RESTART on signal handlers
• feature: added the sig_restart API to the ngx.process module. lua-resty-core#183 Implement the Lua sig_restart() API
• tests: added more test cases for retries on interrupted file I/O. resty-cli#41 Enable the SA_RESTART flag on interrupting signals

If all PRs are merged:

• we get rid of the "retry on EINTR" LuaJIT patch which was refused upstream
• resty-cli users can safely use APIs like io.popen() popen crashes in resty but not in luajit resty-cli#35
• OpenResty users can toggle this behaviour on/off as desired (off by default)