powerd(8): allow to force a method of battery state query

This change allows to determine power source via sysctl or /dev/apm
when devd(8) is running (used by default).

Based on patch from PR; other changes on top of it:

• '-f' (force) -> '-s' (source) parameter renaming;
• allow 'apm' -> 'devd' transition when '-s devd' is set

(if APM is enabled);

• man page update.

Checked on amd64 with -s devd / sysctl and apm
(an extra build with forced USE_APM define set was done)

PR: 125707
Submitted by: Konstantin Stepanov <milezv@yandex.ru>
Reviewed by: bcr, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18742