libdtrace: get rid of illumos ifdefs in dt_module_update(), fix dm_file and dm_modid

Because dt_module_update() is highly OS-specific, the ifdefs make it
hard to read and follow what is going on. Also handle dm_modid, and
remove handling of the ".filename" section, since we can easily fetch
the filename from the module's pathname (k_stat->pathname).

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39177