wifischedule: completely overhaul the script

wifischedule: completely overhaul the script

Squashed using these commits:

 - wifischedule: use `service` instead of direct path
 - wifischedule: use `sort -u` instead of `uniq`
 - wifischedule: restart cron only at the end of batch instead of after every change
 - wifischedule: remove `[[` bash-isms
 - wifischedule: trim trailing ws
 - wifischedule: reduce `if` blocks
 - wifischedule: quote variables and remove some more bash-isms
 - wifischedule: simplify _get_uci_value`
 - wifischedule: don't exit whole script just because `uci get somekey` fails somewhere
 - wifischedule: revamp `_should_enable_wifi()`
 - wifischedule: revamp `_format_daysofweek_list()`
 - wifischedule: revamp `_enable_wifi_schedule()`
 - wifischedule: minor refactoring
 - wifischedule: mega revamp
 - wifischedule: fixes
 - wifischedule: touch-ups
 - wifischedule: use only `awk` in `_cfg_list_entries()` to filter `uci`
 - wifischedule: improve code docs
 - wifischedule: inline `_crontab_format_dow_field()`
 - wifischedule: refactor `_crontab_append_line()`
 - wifischedule: add `_uci_bool()` and refactor `_arith_bool()`
 - wifischedule: rename some functions
 - wifischedule: refactor using shellcheck
 - wifischedule: refactor `_wifi_get_interfaces()`
 - wifischedule: refactor `_wifi_get_devices()`
 - wifischedule: shellcheck fixes
 - wifischedule: use logger instead of a logfile
 - wifischedule: refactor global consts
 - wifischedule: introduce main() func
 - wifischedule: bump version

Signed-off-by: Jan Chren ~rindeal <dev.rindeal@gmail.com>