verify that ttyname refers to the same file as the fd

linux containers use separate mount namespace so the /proc
symlink might not point to the right device if the fd was
opened in the parent namespace, in this case return ENOENT.

include cleanups: remove unused headers and add feature test macros

debloat code that depends on /proc/self/fd/%d with shared function

I intend to add more Linux workarounds that depend on using these
pathnames, and some of them will be in "syscall" functions that, from
an anti-bloat standpoint, should not depend on the whole snprintf
framework.

fix broken ttyname[_r] (failure to null-terminate result)

initial check-in, version 0.5.0