src/common/buf.c: enhance the buffer API

src/common/buf.c: enhance the buffer API

There is at least one user of the buffer API that reuse a single
buffer by just resetting `buf.len` to `0`. This works as long as
the new user of the buffer actually adds something to the buffer.

However, if we don't add anything but still provide `buf.buf` to
a consumer, the old content will be re-used.

This patch thus adds two new clearing variants to the buffer API:
- `buf_clear()` which doesn't reset the internal allocations
- `buf_reset()` which does free the internal allocations

Additionally, this patch makes `buffer_add_char()` public which
allows adding single characters to an existing buffer. This will
be used in a future PR which implements custom format strings for
the OSD.