lib/posix-process: Init signal files ctx on signal desc creation

lib/posix-process: Init signal files ctx on signal desc creation

We have two initialization places for the signal files context of a
signal files context:
- on vfork when a child process is born and its signal descriptor is
correspondingly inherited from the parent
- on signal descriptor initialization on regular process creation

For the former, at this time, we do not support proper open file
inheritance across vfork/execve for those that do not have the
O_CLOEXEC flag. Therefore, for now, simply just initialize child
process' signal descriptor signal file context as empty.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1619