lib/posix-process: Update semantics of vfork()'s return type

lib/posix-process: Update semantics of vfork()'s return type

Although with the current implementation the returned value is identical,
vfork() returns the child's pid rather than tid. Updadte the naming of
identifiers to reflect that correctly.

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1627