tweak OSMO_STRBUF_APPEND(), add OSMO_STRBUF_APPEND_NOLEN()

In OSMO_STRBUF_APPEND, use local variable names that are less likely to shadow
other local variables: prefix with _sb_.

In OSMO_STRBUF_APPEND, add a check to add to .pos only if it is not NULL.

Add OSMO_STRBUF_APPEND_NOLEN(), which works for function signatures that don't
return a length. This is useful for any osmo_*_buf() string writing functions,
so that these write directly to the strbuf.

Change-Id: I108cadf72deb3a3bcab9a07e50572d9da1ab0359
diff --git a/tests/utils/utils_test.ok b/tests/utils/utils_test.ok
index 5783eb1..587c6f0 100644
--- a/tests/utils/utils_test.ok
+++ b/tests/utils/utils_test.ok
@@ -341,6 +341,11 @@
 T minus 10 9 8 7 6 5 4 3 2 1 ... Lift off! -- T minus 10 9 8 7 6 5 4 3 2 1 ... Lift off! -- T minus 10 9 8 7 6 5 4 3 2 1 ... Lift off!
 (need 134 chars, had size=63) T minus 10 9 8 7 6 5 4 3 2 1 ... Lift off! -- T minus 10 9 8 7
 
+strbuf_test_nolen
+20: 0001011100101010 (need=16)
+more: 0001011100101010000 (need=19)
+10: 000101110 (need=9)
+
 startswith_test()
 osmo_str_startswith(NULL, NULL) == true
 osmo_str_startswith("", NULL) == true