New helper function, popenf(), from the Infix project

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/Makefile.am

@@ -10,7 +10,7 @@ libite_la_SOURCES     = chomp.c		copyfile.c			\
 			fparseln.c	fsendfile.c			\
 			ifconfig.c	lfile.c				\
 			makepath.c	progress.c			\
-			pidfile.c	pidfilefn.c			\
+			pidfile.c	pidfilefn.c	popenf.c	\
 			reallocarray.c	rsync.c		strtrim.c	\
 			strlcpy.c 	strlcat.c 	strtonum.c	\
 			strdupa.h	strndupa.h	strnlen.h	\