page eviction and simple mm service routine

[wjhun]

Add drain function to pagecache to evict cached pages. The mm_service routine, called from runloop_internal, simply checks if free physical memory dips below a threshold (CACHE_DRAIN_CUTOFF) and calls the pagecache drain with the delta.

Clearly more can be done to address low memory situations (#1130). Perhaps we could chain drain functions for various caches in the system (of varying types - including object caches) and drain according to some priority scheme. And a low memory threshold could be something more intelligent - a proportion of total memory or something determined dynamically. But a fixed threshold is better than not draining the pagecache at all.

There isn't a runtime test to stress the cache or page eviction. To test these changes, I raised CACHE_DRAIN_CUTOFF to various levels (up to system memory) to force drainage while running 'make test' and a go webserver serving large files.

Also fixes a missing sg_buf release in sendfile(2).

Resolves #1117, partially addresses #1130