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

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.