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>
sheep: fix memory leak in _invert_mat()

First run swift interface on sheepdog, thean kill a node and wait for
recovery. After it, the RSS of sheep daemon will increase to about 8GB
which is a sign of memory leak.

Then we use valgrind to check sheepdog daemon and finally find out the
leak point: _invert_mat() in lib/fec.c

Signed-off-by: Robin DOng <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: fix memory leak in local_req_async_main()

We find out a memory-leak point in local_req_async_main() by valgrind.
After fix it, the RSS of sheep daemon could keep stablely for press-testing.

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