built_in: improved to define DEFFILEMODE if not provided by the libc

musl libc doesn't provide the non-standard macros for common mode
bit masks, which other libc implementations like glibc define in
<sys/stat.h>.

Compile time error when building against musl libc:

--------------------------------------------------------------------------
netsniff-ng.c: In function 'read_pcap':
netsniff-ng.c:592:33: error: 'DEFFILEMODE' undeclared (first use in this function)
--------------------------------------------------------------------------

This change improves built_in.h to check if DEFFILEMODE is defined
and if not it defines it to be available internally.

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
docs: be more precise on readme with experimental tools

Just improve the note on tools that are still experimental.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>