Charge [stack] at its grown extent, as Linux does #492

elfuse maps [stack] at its full fixed extent up front, and #485 counts it that way toward RLIMIT_AS. Linux counts the stack VMA only as far as it has grown.

Measured with one static probe that prints the [stack] line of /proc/self/maps and VmSize from /proc/self/status, right after exec:

So a guest whose ulimit -v was sized from a Linux run gets about 8 MiB less address space under elfuse. test-rlimit-mem cannot see the gap, because it takes its base from elfuse's own VmSize.

To match Linux, [stack] would be charged (and shown in maps) at the extent the stack has grown to, and grown on a fault below it the way Linux expands the stack VMA, with RLIMIT_STACK and RLIMIT_AS checked at that point as acct_stack_growth does.

Refs #485