seq(1): Put separator only between the elements.

• Using non-default ('\n') separator will produce an output with the separator at the end of the output, eg.

% echo "[$(seq -s ' ' 0 2)]"
[0 1 2 ]

• The output should always be followed by a new line character. Currently:

% seq -s ' ' 0 2
0 1 2 %

This change makes seq(1) to behave the same way Linux seq(1):

% echo "[$(seq -s ' ' 0 2)]"
[0 1 2]

% seq -s ' ' 0 2
0 1 2
%

Approved by: oshogbo
Differential Revision: https://reviews.freebsd.org/D43094