logger: use ARG_MAX for limit of command line length

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
logger: prohibit inlining sd_backtrace

This is necessary because sd_backtrace assumes that the top of the
stack frames belongs to itself.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
logger: dump stack frames when showing backtrace

This dumps all function arguments and local variables for each stack
frame using GDB.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: dump cluster info when exiting unexpectedly

This introduces sd_dump_variable to dump a variable and call it
against __sys in crash_handler.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>