plugin/base: improvements (#1930)

* plugins/base: code style improvements

Quote variables, use $@ and $array[@] instead of $*, typeset some integers, remove unneccesasary binary invocation, use shell features when possible, remove `eval`, &c.

* plugins/base: conditional function definitions

Instead of functions failing when required tools aren't installed, just don't define the function.
Alsö, don't redefine del() if it already exists.

* plugins/base: rewrite `usage()`

Reimplement disk usage function using Bash syntax and simpler layout, without having to invoke an external binary.

* plugins/base: revamp `quiet()`

New implementation that is even quieter.

* plugins/base: `myip()`

* plugins/base: `pickfrom()`

* plugins/base: `passgen()`

Fix `passgen()` to not need `tr`, remove one subshell, and eliminate a useless `echo`.

* plugins/base: `mkcd()`

* plugins/base: `mkiso()`

* plugins/base: remove `banish-cookies()`

Adobe Flash is gone with the wind. Alsö, this would be something someone would do *once* and shouldn't be a function...

* plugins/base: `lsgrep` is SC2010

The `lsgrep()` function is *itself* explicitly forbidden by `shellcheck` rule SC2010.

Alsö, s/`$*`/`$@`

* plugins/base: `mkiso()`

Expressly handle unbound parameters.

* plugins/base: remove `command_exists`

* plugin/base: lint SC2154 && SC2144

Newly undisabled `shellcheck` rules

* plugin/base: import libs for tests

* plugin/base: `shfmt`

Apply `shfmt` using current project settings. My apologies to future `git blame` hunters. ♥

test/plugins/base.plugin.bats

@@ -2,6 +2,8 @@
 
 load ../test_helper
 load "${BASH_IT}/vendor/github.com/erichs/composure/composure.sh"
+load ../../lib/log
+load ../../lib/helpers
 load ../../plugins/available/base.plugin
 
 @test 'plugins base: ips()' {