Portability: remove dependency on a particular set of operating systems.

The initial BSD version was too BSD-specific, which necessitated addition
of the Linux branch. However, the Linux branch is actually more portable,
so we leave it here, removing the BSD one.
Remove dependency on librt.

librt is used for clock_gettime(), which can be emulated by gettimeofday()
without any percieved downside. librt dependency is not cross-platform.
avoid dying on SIGPIPE
ignore resulting binary
reverted debug CFLAGS
remove unused function
Do not send Connection: keep-alive

HTTP/1.1 has keep-alive by default, no need to send Connection: keep-alive
header when `-k` is given.
cleanup keepalive logic (remove goto)
Report system limits to the user.

Avoid SIGSEGV if we can't get what we want. Be proactive in discovering limits
Merge pull request #4 from vlm/vlm/fix-rlimit

Report system limits to the user.
Avoid being anal for keep-alive.
Mac os x port