Capsicumize savecore(8).

• Use cap_fileargs(3) to open dump devices after entering capability mode, and use cap_syslog(3) to log messages.
• Use a relative directory fd to open output files.
• Use zdopen(3) to compress kernel dumps in capability mode.

Reviewed by: cem, oshogbo
MFC after: 2 months
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18458