ln(1): fix -F behavior

When '-F' option is used, the target directory needs to be unlinked.
Currently, the modified target ("target/source") is being unlinked, and
since it doesn't yet exist, the original target isn't removed.
This is fixed by skipping the block where target is modified to
"target/source" when '-F' option is set.
Hence, a symbolic link (with the same name as of the original target) to
the source_file is produced.

Update the test for ln(1) to reflect fix for option '-F'

MFC after:	1 month
PR:		219943
Differential Revision:	D11167
Submitted by:	shivansh
Sponsored by:	Google (GSoC 2017)

bin/ln/ln.c

@@ -245,11 +245,11 @@ linkit(const char *source, const char *target, int isdir)
 
 	/*
 	 * If the target is a directory (and not a symlink if hflag),
-	 * append the source's name.
+	 * append the source's name, unless Fflag is set.
 	 */
-	if (isdir ||
+	if (!Fflag && (isdir ||
 	    (lstat(target, &sb) == 0 && S_ISDIR(sb.st_mode)) ||
-	    (!hflag && stat(target, &sb) == 0 && S_ISDIR(sb.st_mode))) {
+	    (!hflag && stat(target, &sb) == 0 && S_ISDIR(sb.st_mode)))) {
 		if (strlcpy(bbuf, source, sizeof(bbuf)) >= sizeof(bbuf) ||
 		    (p = basename(bbuf)) == NULL ||
 		    snprintf(path, sizeof(path), "%s/%s", target, p) >=