Fix various Coverity issues in sbin/savecore/savecore.c:

CID1009429: Fix unchecked return value from lseek while clearing dump
CID1007781: Fix file descriptor leak in DoFile
CID1007261: Don't send potentially unterminated string to syslog(3)

Coverity CID: 1009429
Coverity CID: 1007781
Coverity CID: 1007261
MFC after: 2 weeks
Sponsored by: Spectra Logic
Differential Revision: https://reviews.freebsd.org/D3991