Fix bugs in FreeBSD version of setuser() and setgroup().

Signed-off-by: Sachin Bhamare <sbhamare@panasas.com>
Fix the order in which credentials are set in fsal_set_credentials().

On FreeBSD, set_threadgroups() translates to setgroups system call which only a super user
can issue. Current sequence in fsal_set_credentials() is
setuser-->setgroup-->set_threadgroups which doesnt work, atleast, on FreeBSD
platform.

Signed-off-by: Sachin Bhamare <sachin.bhamare@gmail.com>