IMPORT: ebtree: make string_equal_bits() return an unsigned

IMPORT: ebtree: make string_equal_bits() return an unsigned

It used to return ssize_t for -1 but in fact we're using this -1 as
the largest possible value and the result is generally cast to signed
to check if the end was reached, so better make it clearly return an
unsigned value here.

This is cbtree commit e1e58a2b2ced2560d4544abaefde595273089704.
This is ebtree commit d7531a7475f8ba8e592342ef1240df3330d0ab47.