basic: add hmac_sha256 implementation

Based on the FIPS 198 specification. Not optimized and probably
completely unsafe, to be used only for non-strong-cryptographic
purposes when OpenSSL cannot be used.

src/basic/meson.build

@@ -72,6 +72,8 @@ basic_sources = files('''
         hashmap.h
         hexdecoct.c
         hexdecoct.h
+        hmac.c
+        hmac.h
         hostname-util.c
         hostname-util.h
         in-addr-util.c