MINOR: tools: add strordered() to check whether strings are ordered

MINOR: tools: add strordered() to check whether strings are ordered

When trying to sort sets of strings, it's often needed to required to
compare 3 strings to see if the chosen one fits well between the two
others. That's what this function does, in addition to being able to
ignore extremities when they're NULL (typically for the first iteration
for example).