bugfix: lib/sh/util.sh: become_sudo don't continue running function after self_sudo finishes

bugfix: lib/sh/sync.sh, db.sh: *_snapshot() commands that run multiple sudo commands: use become_sudo so that the program won't hang at later sudo prompts. for pg_snapshot() with pg_start_backup(), this also prevents the backup from staying open at the end waiting on a sudo prompt, causing excess WAL logs to accrue.

lib/sh/util.sh: become_sudo: documented that this is for funcs that run multiple sudo commands

lib/sh/util.sh: $sudo_cmd_wrapper: use esc() so this does not need to be embedded with [@] array notation

bugfix: lib/runscripts/util.run: $wrap_fn: this always needs to contain `is_runscript=1` because $is_runscript is not (and cannot be) exported

bugfix: lib/sh/util.sh: sudo(): when don't need sudo: do need `command` to have effects of invoking external command

fix: lib/sh/db.sh: pg_snapshot(): first need to run pg_stop_backup() in case a backup was not stopped cleanly

fix: lib/sh/db.sh: pg_stop_backup(): ignore benign "a backup is not in progress" error

lib/sh/util.sh: sudo(): only run sudo if needed

lib/sh/util.sh: sudo(): factored PATH-preserving code out into separate wrapper var that can also be used separately, eg. with `sudo -u`