sheep: disable glibc's dynamic mmap threshold

This solves the problem that freed memory is retained in the long running sheep.

This is due to glibc's memory allocation implementation (ptmalloc). For 
allocations greater than or equal to the limit specified by M_MMAP_THRESHOLD 
that can't be satisfied from the free list, the memory-allocation functions
employ mmap(2) instead of increasing the program break using sbrk(2).

So for the long running sheep, 
1 M_MMAP_THRESHOLD is increased to the extent that all the allocation use its
  internal cached chunks on the heap.
2 ptmalloc can't release the memory of sub-heap back if top chunk is in use.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>