unwind: introduce markers specifying the needs of special care in unwinding

unwind: introduce markers specifying the needs of special care in unwinding

Some system calls require capturing the stack trace before they are
processed in kernel.  Typical one is execve.  Some system calls require
invalidating mmap cache after they are processed in kernel.

In current implementation these requirements are handled directly by
appropriate syscall handlers.  However, it is difficult to keep the
source code maintainable using this approach to cover all system calls
which have such requirements.

A more generic way to implement this is to flag all syscalls that
require special processing, and handle these flags right in
trace_syscall_entering instead of changing syscall handlers.

This patch just defines new flags: STACKTRACE_INVALIDATE_CACHE and
STACKTRACE_CAPTURE_ON_ENTER.

The names of macros are suggested by Dmitry Levin.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>