syslogd: Create syslogd libcasper service

Once in capability mode, resource acquisition is not possible. To
mitigate this, introduce a libcasper service that is responsible for
fetching and returning new, requested resources to syslogd.

Some resources must be packed into an nvlist to be properly transferred
between syslogd and the libcasper process. The filed_to_nvlist() and
nvlist_to_filed() functions are included to ease the packing process for
filed structures.

Two additional syslogd.h and syslogd_cap.h header files are included
with shared declarations.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41463