exterror(9): dynamic kernel categories

Make it possible to define categories without compiling their
paths into libc (important for third-party modules). The
EXTERR_CATEGORY_DYNAMIC macro can be defined to a string describing the
compilation unit (generally the path relative to src/sys) which takes
the place of EXTERR_CATEGORY.

These strings are assembled in linker sets with category numbers
assigned at system startup or module load time. The strings can be
retrieved from the kern.exterr.categories.<category> sysctl.

Reviewed by: kib
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D58237