ubox: support alternative spellings for modinfo and rmmod

After this change, "rmmod" and "modinfo" works even if the user gives
the file name spelling instead of the kernel internal one, so that e.g.
"rmmod nls_iso8859-1.ko" and "rmmod nls_iso8859_1" will both succeed.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38252 3c298f89-4303-0410-b956-a3cf2f4a3e73