Feature request: Add support for saving `errno` after foreign calls

[spcfox]

After calling foreign procedures that use system calls, it may be useful to retrieve errno. According to this comment, retrieving it via a separate call is unsafe, since errno may be modified by the Chez runtime. It would be useful to have an analogue of Racket’s #:save-errno option.

[mflatt]

For what it's worth, I expect to work on this in a few weeks if no one gets to its sooner. Meanwhile, you might try the same strategy that Racket uses, which is to disable interrupts around the foreign call and errno fetch.

[amirouche]

@spcfox see #550 (comment)

[burgerrg]

Fixed in #1013