Uses channame() in sys_fd2path()

channame() should do the trick, instead of the old chanpath() which was
removed in commit 011ab2.  If not, we should fix channame().  chanpath()
was a nxm thing; Inferno has different structs to track the name.
"chanpath(ch)" might have been put in quotes during the 9ns->inferno
change.

Using snprintf() and channame() helps with a couple things.  Previously,
fd2path() didn't check ch != 0 and could copy more than the user requested
(i.e. the full channame len instead of MIN(len, ch->name->len)).