osdep/io: also include unistd.h

Might be needed by fcntl() usage.
old-configure: delete pre-waf config.h and config.mak
options: add options that set defaults for af/vf/ao/vo

There are some use cases for this. For example, you can use it to set
defaults of automatically inserted filters (like af_lavrresample). It's
also useful if you have a non-trivial VO configuration, and want to use
--vo to quickly change between the drivers without repeating the whole
configuration in the --vo argument.