Add unescape() to libutil

formerly known as resolveescapes(), it is of central use to numerous
programs.
This drops a lot of LOC.

libutil/unescape.c

@@ -0,0 +1,50 @@
+/* See LICENSE file for copyright and license details. */
+#include <string.h>
+
+#include "../util.h"
+
+size_t
+unescape(char *s)
+{
+	size_t len, i, off, m, factor, q;
+
+	len = strlen(s);
+
+	for (i = 0; i < len; i++) {
+		if (s[i] != '\\')
+			continue;
+		off = 0;
+
+		switch (s[i + 1]) {
+                case '\\': s[i] = '\\'; off++; break;
+                case 'a':  s[i] = '\a'; off++; break;
+                case 'b':  s[i] = '\b'; off++; break;
+                case 'f':  s[i] = '\f'; off++; break;
+                case 'n':  s[i] = '\n'; off++; break;
+                case 'r':  s[i] = '\r'; off++; break;
+                case 't':  s[i] = '\t'; off++; break;
+                case 'v':  s[i] = '\v'; off++; break;
+		case '\0':
+			eprintf("%s: null escape sequence\n", argv0);
+		default:
+			/* "\O[OO]" octal escape */
+			for (m = i + 1; m < i + 1 + 3 && m < len; m++)
+				if (s[m] < '0' || s[m] > '7')
+					break;
+			if (m == i + 1)
+				eprintf("%s: invalid escape sequence '\\%c'\n", argv0, s[i + 1]);
+			off += m - i - 1;
+			for (--m, q = 0, factor = 1; m > i; m--) {
+				q += (s[m] - '0') * factor;
+				factor *= 8;
+			}
+			s[i] = q;
+		}
+
+		for (m = i + 1; m <= len - off; m++)
+			s[m] = s[m + off];
+		len -= off;
+	}
+
+	return len;
+}