[wmii] Small bugs in fs.c (patch)

When compiling wmii under OS X (Intel) I found a segfault in
cmd/wm/fs.h. The OS X linker is confusing the "err" symbol in
cmd/wm/fs.h with the "err" function in libc. Attached is a patch to
fix that as well as a few off-by-one bounds-check errors I found while
tracking it down. Otherwise wmii compiles and runs beautifully under
OS X; great job Anselm and everyone!