Print enabled optional features in strace version output
In order to provide information to user what optionally built features
are available.
* strace.c (print_version): New variable "features". Print features string
after non-liability disclaimer (or "(none)" in case it is empty).
(print_version) [USE_LIBUNWIND]: Concatenate "stack-unwind" into features
string.
* tests/strace-V.tests (getoption): New function.
Update check in accordance with updated output.