include cleanups: remove unused headers and add feature test macros

more close-on-exec fixes, mostly using new "e" flag to fopen

cleanup src/linux and src/misc trees, etc.

previously, it was pretty much random which one of these trees a given
function appeared in. they have now been organized into:

src/linux: non-POSIX linux syscalls (possibly shard with other nixen)
src/legacy: various obsolete/legacy functions, mostly wrappers
src/misc: still mostly uncategorized; some misc POSIX, some nonstd
src/crypt: crypt hash functions

further cleanup will be done later.