[DRE-maint] Bug#988091: batalert: Fails to start because BAT0 is assumed

[Pelle]

Package: batalert
Version: 0.4.0-1
Severity: important

Dear Maintainer,

I get this error when running batalert:

```
$ batalert
Traceback (most recent call last):
        5: from /usr/bin/batalert:23:in `<main>'
        4: from /usr/bin/batalert:23:in `load'
        3: from /usr/share/rubygems-integration/all/gems/batalert-0.4.0/exe/batalert:5:in `<top (required)>'
        2: from /usr/share/rubygems-integration/all/gems/batalert-0.4.0/lib/batalert.rb:14:in `main'
        1: from /usr/share/rubygems-integration/all/gems/batalert-0.4.0/lib/batalert.rb:14:in `open'/usr/share/rubygems-integration/all/gems/batalert-0.4.0/lib/batalert.rb:14:in `initialize': No such file or directory @ rb_sysopen - /sys/class/power_supply/BAT0/capacity (Errno::ENOENT)
```

This appears to be caused by batalert making an incorrect assumption 
about the battery name:

```
$ ls /sys/class/power_supply/
ACAD  BAT1
```

I expected batalert to either correctly detect the device battery name or at least to provide a flag for setting it.

-- System Information:
Debian Release: 11.0
  APT prefers unstable-debug
  APT policy: (500, 'unstable-debug'), (500, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 5.10.0-6-amd64 (SMP w/2 CPU threads)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_US:en
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages batalert depends on:
ii  ruby            1:2.7+2
ii  ruby-espeak     1.0.4-2
ii  ruby-libnotify  0.9.4-3

batalert recommends no packages.

batalert suggests no packages.

-- no debconf information