sys: add conf/std.debug, generic debugging options

The new sys/conf/std.debug contains the list of debugging options
enabled by default in -CURRENT, so they don't need to be listed
individually in every kernel config.

Introduce *-DEBUG variants of the major kernel configs.

(cherry picked and modified from commit 4f8f9d708e6a4143f3b178bfab10d0a9b75ba2fe)

Reviewed by: markj, imp, olce (previous version)
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46871