ci: Add ci.lib.ch; update ci_verify_*.sh accordingly

Move the common declarations and initializations from ci_verify_*.sh
to ci.lib.sh, and update them as follows:
 * Simplify the ci_ function names.
 * Refactor the CI_ variable names:
    - Add the new variable CI_TOPLEVEL_DIR.
    - Rename the variables CI_SCRIPTNAME, CI_SCRIPTDIR, etc.,
      to CI_SCRIPT_NAME, CI_SCRIPT_DIR, etc.
    - Rename the variables CI_SRCDIR_FROM_BUILDDIR, etc., to
      CI_BUILD_TO_SRC_RELDIR, etc.
 * Add new functions inside ci.lib.sh:
    - Replace ci_err with ci_err_usage, ci_err_fatal, ci_err_internal.
    - Add the new functions ci_warn and ci_assert.
 * Simplify the ci_ function names inside ci_verify_*.sh.
4 files changed