audit: allow other filter list types for AUDIT_EXE

This patch removes the restriction of the AUDIT_EXE field to only
SYSCALL filter and teaches audit_filter to recognize this field.

This makes it possible to write rule lists such as:

    auditctl -a exit,always [some general rule]
    # Filter out events with executable name /bin/exe1 or /bin/exe2:
    auditctl -a exclude,always -F exe=/bin/exe1
    auditctl -a exclude,always -F exe=/bin/exe2

See: https://github.com/linux-audit/audit-kernel/issues/54



Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>