common: remove buf_init(), add BUF_INIT and buf_move()

Add a BUF_INIT macro, which makes it easier to initialize a struct buf
to an empty string (without a heap allocation).

Add buf_move() to move the contents of one struct buf to another (the
source is reset to BUF_INIT, analogous to C++ move-assignment).

Use buf_reset() instead of directly calling `free(s->buf)` since the
internal buf may not always be allocated by malloc() now.

src/config/session.c

@@ -49,13 +49,12 @@ process_line(char *line)
 		return;
 	}
 
-	struct buf value;
-	buf_init(&value);
+	struct buf value = BUF_INIT;
 	buf_add(&value, string_strip(++p));
 	buf_expand_shell_variables(&value);
 	buf_expand_tilde(&value);
 	setenv(key, value.buf, 1);
-	free(value.buf);
+	buf_reset(&value);
 }
 
 /* return true on successful read */