Function: sh-guess-basic-offset

See if we can determine a reasonable value for `sh-basic-offset'.
This is experimental, heuristic and arbitrary!
Argument VEC is a vector of information collected by
`sh-learn-buffer-indent'.
Return values:
number - there appears to be a good single value
list of numbers - no obvious one, here is a list of one or more
reasonable choices
nil - we couldn't find a reasonable one.