libutil: Add writeall utility function

writeall makes successive write calls to write an entire buffer to the
output file descriptor. It returns the number of bytes written, or -1 on
the first error.

Makefile

@@ -79,7 +79,8 @@ LIBUTILSRC =\
 	libutil/strlcpy.c\
 	libutil/strsep.c\
 	libutil/strtonum.c\
-	libutil/unescape.c
+	libutil/unescape.c\
+	libutil/writeall.c
 
 LIB = $(LIBUTF) $(LIBUTIL)
 

libutil/writeall.c

@@ -0,0 +1,21 @@
+/* See LICENSE file for copyright and license details. */
+#include <unistd.h>
+
+#include "../util.h"
+
+ssize_t
+writeall(int fd, const void *buf, size_t len)
+{
+	const char *p = buf;
+	ssize_t n;
+
+	while (len) {
+		n = write(fd, p, len);
+		if (n <= 0)
+			return n;
+		p += n;
+		len -= n;
+	}
+
+	return p - (const char *)buf;
+}

util.h

@@ -62,6 +62,9 @@ char *strsep(char **, const char *);
 int enregcomp(int, regex_t *, const char *, int);
 int eregcomp(regex_t *, const char *, int);
 
+/* io */
+ssize_t writeall(int, const void *, size_t);
+
 /* misc */
 void enmasse(int, char **, int (*)(const char *, const char *, int));
 void fnck(const char *, const char *, int (*)(const char *, const char *, int), int);