libbb: add optionl support for SHA256/512 encrypted passwords

function                                             old     new   delta
sha_crypt                                              -    2423   +2423
cryptpw_main                                         128     183     +55
to64                                                   -      29     +29
pw_encrypt                                           974    1000     +26
str_rounds                                             -      11     +11
login_main                                          1532    1541      +9
packed_usage                                       25215   25200     -15
__md5_to64                                            29       -     -29
------------------------------------------------------------------------------
(add/remove: 3/1 grow/shrink: 3/1 up/down: 2553/-44)         Total: 2509 bytes

diff --git a/libbb/pw_encrypt_md5.c b/libbb/pw_encrypt_md5.c
index 8d0a516..b7478aa 100644
--- a/libbb/pw_encrypt_md5.c
+++ b/libbb/pw_encrypt_md5.c
@@ -494,16 +494,6 @@
 }
 
 
-static char*
-__md5_to64(char *s, unsigned v, int n)
-{
-	while (--n >= 0) {
-		*s++ = ascii64[v & 0x3f];
-		v >>= 6;
-	}
-	return s;
-}
-
 /*
  * UNIX password
  *
@@ -605,9 +595,9 @@
 	final[16] = final[5];
 	for (i = 0; i < 5; i++) {
 		unsigned l = (final[i] << 16) | (final[i+6] << 8) | final[i+12];
-		p = __md5_to64(p, l, 4);
+		p = to64(p, l, 4);
 	}
-	p = __md5_to64(p, final[11], 2);
+	p = to64(p, final[11], 2);
 	*p = '\0';
 
 	/* Don't leave anything around in vm they could use. */