hmac: Implementation of HMAC in glib

This implements g_hmac_xxx() functionality using the standard checksum
functions supported by glib.

HMAC is a secure way to hash a key and a password. Many other
approaches fraught with append and prepend issues.

Includes test cases defined in relevant RFCs

https://bugzilla.gnome.org/show_bug.cgi?id=652480

docs/reference/glib/glib-sections.txt

@@ -2828,6 +2828,22 @@ g_compute_checksum_for_data
 g_compute_checksum_for_string
 </SECTION>
 
+<SECTION>
+<TITLE>Data HMACs</TITLE>
+<FILE>hmac</FILE>
+GHmac
+g_hmac_new
+g_hmac_copy
+g_hmac_ref
+g_hmac_unref
+g_hmac_update
+g_hmac_get_string
+g_hmac_get_digest
+<SUBSECTION>
+g_compute_hmac_for_data
+g_compute_hmac_for_string
+</SECTION>
+
 <SECTION>
 <TITLE>Testing</TITLE>
 <FILE>testing</FILE>