Add qemu_strndup: qemu_strdup with length limit.

Also optimise qemu_strdup by using memcpy - using pstrcpy is usually 
suboptimal.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5653 c046a42c-6fe2-441c-8c8c-71466251a162
