newsyslog(8): Implement a new 'E' flag to not rotate empty log files

Based on an idea from dvl's coworker, László DANIELISZ, implement
a new flag, 'E', that prevents newsyslog(8) from rotating the empty
log files. This 'E' flag ist mostly usable in conjunction with 'B'
flag that instructs newsyslog(8) to not insert an informational
message into the log file after rotation, keeping it still empty.

Reviewed by: markj, ian, manpages (rpokala)
Approved by: markj, ian, manpages (rpokala)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28940