Make -e fault= expressions cumulative

Make -e fault= expressions cumulative

Change the way how subsequent -e fault= expressions are interpreted
to implement a cumulative behavior.  For example,
-e fault=file:when=3+ -e fault=chdir
now specifies that all chdir syscalls and 3+ file related syscalls
except chdir are subject for fault injection.

* syscall.c (qualify): Do not reset qual_vec for QUAL_FAULT.
* tests/fault_injection.test: Check it.