setpriv: avoid alloca() use xmalloc() instead

setpriv: avoid alloca() use xmalloc() instead

The getgroups() can return up to NGROUPS_MAX supplementary groups, that
is (since kernel 2.6.3) 65536 in total.  The git_t is 4 bytes, so maximum
request is 256 kilobytes.  When a system happen to have memory preasure
alloca() may not be able to allocate enough memory, making debugging
unnecessarily difficult.  IMHO 64 pages is significant enough amount of
memory to be properly error checked at a time of allocation.

Reference: http://www.gnu.org/software/libc/manual/html_node/Disadvantages-of-Alloca.html
Signed-off-by: Sami Kerola <kerolasa@iki.fi>