Add "flags" field to struct inject_data

Add "flags" field to struct inject_data

Introduce "flags" field to struct inject_data and use it instead
of magic rval and signo constants.
Due to layout of struct inject_data, this new field does not change
sizeof(struct inject_data).

* defs.h (INJECT_F_RETVAL, INJECT_F_SIGNAL): New macros.
(INJECT_OPTS_RVAL_DEFAULT): Remove macro.
(struct inject_data): Add "flags" field.
* filter_qualify.c (parse_inject_token, qualify_inject_common): Check
struct inject_data.flags instead of inject_data.rval
and inject_data.signo, do not initialize inject_opts.data.
* syscall.c (tamper_with_syscall_entering): Check struct
inject_data.flags instead of inject_data.rval and inject_data.signo.