ipc: add string transfer test `echo_with_long_hole'

It puts the send and receive buffers across a page boundary, of which either
side is unmapped. The test could be further refined to selectively leave
first/second halves of the send/receive sides mapped to test out the "complete
memcpy_from across pages" path as well.
ipc: fix stritem_faults() in the non-aligned missing entry case
ipc: send next prexfer fault

Also move the n_srcf, n_dstf shit into struct fault_peer, which is far nicer.
testbench: move the stats pager out of space_suite.c

Also move send_quit() into util.c, and use it in string_suite.c.
testbench: use pager stats in echo_with_long_hole

Six more test points.
testbench: move 20_loop.pl to 20_sink.pl

It doesn't actually test Mung::Loop at all.