lib/sh/util.sh: added is_callable()

lib/runscripts/util.run: support custom handlers for all targets (gateway()) as well as targets w/o function (fallback())

lib/runscripts/table.run: remake_VegBIEN_mappings(): renamed to just mappings() since action make targets should be short names

lib/sh/util.sh: stderr_matches(): inline the stderr_matches alias to avoid needing to quote stderr_matches as "stderr_matches" in the most common use case (with pattern as a prefix env var)

bugfix: lib/sh/util.sh: stderr_matches(): when passing `pattern=...` as a prefix env var, must be invoked as `"stderr_matches"` to avoid the env var applying to the prep_try portion of the stderr_matches alias

added schemas/VegCore/Brad_Boyle/bien3_data_provenance_use_cases.docx* from e-mail from Brad

schemas/vegbien.sql: _plots_08_list_of_plots_which_use_percent_cover, _plots_15_pct_cover_of_each_verb_taxon_in_each_plot_in_each_pro: reran with fixes, which removes the incorrectly auto-added copies columns. (they were only able to be auto-added because the tables had no rows.)

bugfix: drop_column(regclass[]): need to run `SELECT NULL::void;` at end of function to avoid folding away functions called in previous query

fix: schemas/util.sql: diff(regclass, regclass): moved try_create() of copies column in parent table to auto_rm_freq() so that it would only happen if both tables actually contain a copies column (otherwise, the try_create() will create an empty copies column if both tables are empty)

schemas/util.sql: try_create(): also handle "child table is missing column" errors