* manual/macros.texi: Introduce macros to document multi
thread, asynchronous signal and asynchronous cancellation
safety properties.
* manual/intro.texi: Introduce the properties themselves.
* manual/argp.texi: Document MTASC-safety properties.
* manual/arith.texi: Document MTASC-safety properties.
* manual/conf.texi: Document MTASC-safety properties.
* manual/crypt.texi: Document MTASC-safety properties.