Fix failing unit tests

Case 114929: Fix a number of failing unit tests, mostly on account of some
bad assumptions made about host environment

Changes:

    * Make t/lib-Archive-Tar-Builder_bigfile.t not run by default on cPanel
      internal machines, ONLY when TEST_BIGFILE=1 set in environment

    * Make t/lib-Archive-Tar-Builder.t use /etc/hosts, not /etc/fstab, as a
      test file; /etc/fstab actually does not exist on some Unixes, like
      Darwin

    * Change b_error_warn() to a function returning true if an error
      object last indicated a warning error, to aid in detecting non-fatal
      warnings which do not completely halt the production of archives
Merge branch 'case114929' of git://deathstar.dev.cpanel.net/~erin/archive-tar-builder/erins-archive-tar-builder