util & scmutil: adapt read/write helpers as request by mpm

store: use a subclass, not a function for fncacheopener

util.termwidth: never return 0 for terminal width Catch a case where the termwidth was being reported as 0 when I was connecting with TLMTask instead of NSTask in OSX. This caused the progress extension to print no progress. The termwidth should never return 0 so in case we would return 0, simply fall back to the default termwidth below which is 80.