(battery-linux-proc-apm): Use string-to-number.

(battery-linux-proc-apm): Use string-to-number.
(battery-linux-proc-acpi): Ignore all hidden files in the battery dir.
Use dolist (and less indentation).  Use erase-buffer.
Pre-initialize `design-capacity', `warn', and `low'.
(battery-format): Use replace-regexp-in-string.