Use fpurge/__fpurge to cancel buffered output

[olebole]

This is an alternative to NOIRLAB IRAFs 48b2f61, which defines fcancel() as setvbuf() on macOS:

iraf/unix/os/zxwhen.c

Lines 90 to 96 in 3761a03

	#ifdef MACOSX
	#define _IONBF 2 /* No buffering. */
	#define fcancel(fp) setvbuf(fp, NULL, _IONBF, 0);
	#else
	//#define fcancel(fp) ((fp)->_r = (fp)->_w = 0)
	#define fcancel(fp)
	#endif

setvbuf() is not what fcancel() is supposed to do, namely to cancel any buffered output. Instead, it just makes the output unbuffered. i.e. when applying this patch, stdout will become unbuffered for the rest of the session after ^C. Also, setvbuf() is not macOS specific but part of the standard C library (and therefore also available for Linux). And, hard-coding _IONBF instead of using the proper include file (stdio.h) is bad style.

The better solution (although non-portable) is to use fpurge() on macOS (and BSD variants), and __fpurge() on Linux (and glibc variants, like HURD). This is done in this PR. While not really portable, these are the libc functions for this task. Linux is covered with the __GLIBC__ macro here, because it is not a Linux but a glibc feature. HURD is handled like Linux.

@mjfitzpatrick, FYI.