Improve the mt(1) rblim display.

The granularity reported by READ BLOCK LIMITS is an exponent, not a
byte value. So a granularity of 0 means 2^0, or 1 byte. A
granularity of 1 means 2^1, or 2 bytes.

Print out the individual block limits on separate lines to improve
readability and avoid exceeding 80 columns.

usr.bin/mt/mt.c:
Fix and improve the 'mt rblim' output. Add a MT_PLURAL()
macro so we can print "byte" or "bytes" as appropriate.

Sponsored by: Spectra Logic
MFC after: 4 days