Add base64.

The tizen guys wanted this. Yeah, I know there's base64 code in
uuencode/uudecode, but that this has -i, input lines aren't of fixed length,
encode/decode are in same file, there's no prefix/suffix code, it always
writes to stdout... Eliminating the code duplication wouldn't be worth
the if/else I'd have to add, so I just did a new one.

Factored out the base64 table init into lib.c though: that was worth sharing.
diff --git a/lib/lib.c b/lib/lib.c
index 33be44b..490235f 100644
--- a/lib/lib.c
+++ b/lib/lib.c
@@ -571,6 +571,21 @@
   }
 }
 
+// Init base64 table
+
+void base64_init(char *p)
+{
+  int i;
+
+  for (i = 'A'; i != ':'; i++) {
+    if (i == 'Z'+1) i = 'a';
+    if (i == 'z'+1) i = '0';
+    *(p++) = i;
+  }
+  *(p++) = '+';
+  *(p++) = '/';
+}
+
 // Quick and dirty query size of terminal, doesn't do ANSI probe fallback.
 // set x=80 y=25 before calling to provide defaults. Returns 0 if couldn't
 // determine size.