Cleans up bind/mount parameter naming

'old' and 'new' are bad names for this stuff.

$ bind src_path onto_path flags

plan9 bind: bind NEW OLD (new == src, old == onto).
linux bind: mount --bind OLD NEW (old == src, new == onto).
Abort sys_open and sys_stat 9ns callouts

Unless the path begins with /9, open and stat will ignore the 9ns code.  This
cuts down on a lot of debugging output.