fix bug in Ctrl-W in xreadline implementation

when there's multiple spaces, the previous logic didn't erase
them, E.g:

    a word   |   < before
    a word  |    < after Ctrl-w

this patch brings the behavior closer to readlines:

    a word   |   < before
    a |          < after Ctrl-w

this also slightly changes the behavior since '/' is no longer
considered a boundary.

src/nnn.c

@@ -3718,13 +3718,13 @@ static char *xreadline(const char *prefill, const char *prompt)
 				continue;
 			case CONTROL('W'):
 				printmsg(prompt);
-				do {
-					if (pos == 0)
-						break;
-					memmove(buf + pos - 1, buf + pos,
-						(len - pos) * WCHAR_T_WIDTH);
-					--pos, --len;
-				} while (buf[pos - 1] != ' ' && buf[pos - 1] != '/'); // NOLINT
+				lpos = pos;
+				while (pos > 0 && ISSPACE_(buf[pos - 1]))
+					--pos;
+				while (pos > 0 && !ISSPACE_(buf[pos - 1]))
+					--pos;
+				memmove(buf + pos, buf + lpos, (len - lpos) * WCHAR_T_WIDTH);
+				len -= lpos - pos;
 				continue;
 			case CONTROL('K'):
 				printmsg(prompt);