lib: fix compiler error in logger.c

When build sheepdog in centos6, it will report:

    logger.c: In function ‘gdb_cmd’:
    logger.c:746: error: ‘ARG_MAX’ undeclared (first use in this function)
    logger.c:746: error: (Each undeclared identifier is reported only once
    logger.c:746: error: for each function it appears in.)
    logger.c:746: warning: unused variable ‘cmd_str’
    logger.c: In function ‘sd_backtrace’:
    logger.c:797: error: ‘ARG_MAX’ undeclared (first use in this function)
    logger.c:797: warning: unused variable ‘cmd’

the reason has been explained here: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=518858
and the standard revolution is using 'sysconf()'.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>

Conflicts:
	lib/logger.c

Conflicts were resolved by Hitoshi Mitake.
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>