Fix `forkProcess` to inherit caller's `MaskingState`

...and while at it, use `bracket` to fix a potential resource leak due
to `freeStablePtr` not being called if `throwErrnoIfMinus1` throws an
exception.

This fixes #8433

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Add `forkProcessWithUnmask` function

This seemed to be an obvious addition while working on #8433.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Fix markup for function names in DEPRECATION messages

This helps Haddock make tose hyperlinked functions.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>