The Linux feature allows writing new files to an invisible file, and
then replace the old file.  This preserves the old file if we get
interrupted by some event.

Fixes a problem with the "curl" input plugin: IsEOF() always returns
true because the "open" flag was cleared by
CurlInputStream::RequestDone() when end-of-stream was reached.  This
flag stays false even when seeking to another position has succeeded.

This patch resets the "open" flag to true after seeking successfully.

NetBSD's pthread_setname_np() prototype is incompatible with the rest
of the world, and it requires to pass the string argument as a
non-const pointer.  Instead of working around this misdesign, I hereby
disable the feature on NetBSD.

Add macro HAVE_THREAD_NAME which is set when any method to set the
thread name is available.  Use that macro in FormatThreadName()
instead of just checking for HAVE_PTHREAD_SETNAME_NP.

Supports "~/" expansion.  Forces us to switch from "const char *" to
AllocatedPath, which is a good thing.