lib/posix-timerfd: Output correct old_value on set

lib/posix-timerfd: Output correct old_value on set

Previously settime() would output the old timerfd setting verbatim, as
an absolute deadline; this contradicts timerfd_settime(2) which clearly
states that old_value should be output with the same semantics as
gettime() -- relative time remaining until the next expiration.
This change makes settime() calculate and output this time correctly.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
GitHub-Closes: #1568