Now that everything uses pthreads and pthreadperthread, can delete various conditionals, all the custom context code, and so on. Also update documents. Fixes #355.

getdirentries(2) has been deprecated on macOS since 10.5 (ten releases ago). Using it requires disabling 64-bit inodes, but that in turn makes binaries incompatible with some dynamic libraries, most notably ASAN. At some point getdirentries(2) will actually be removed. For both these reasons, switch to opendir/readdir. A little clunky since we have to keep the DIR* hidden away to preserve the int fd interfaces, but it lets us remove a bunch of OS-specific code too.

Linux 2.4 is dead. (The libthread code hasn't worked for Linux 2.4 for a long time.)

Pretty sure FreeBSD 4 is gone now. :-)

Since glibc 2.20, the _BSD_SOURCE and _SVID_SOURCE macros are deprecated in favor of the _DEFAULT_SOURCE macro. See https://sourceware.org/glibc/wiki/Release/2.20#Packaging_Changes Change-Id: I18dd6a698f3f5aa51d1e45bf53b031bb061e17e8 Reviewed-on: https://plan9port-review.googlesource.com/1500 Reviewed-by: David du Colombier <0intro@gmail.com>

R=rsc http://codereview.appspot.com/6401049

R=rsc http://codereview.appspot.com/1791044

R=, rsc CC= http://codereview.appspot.com/203061

http://codereview.appspot.com/116096