Increase BUFSIZ to 8192

Hi,

I would like to change the default value of BUFSIZ to 8192.

After testing various applications that uses BUFSIZ like changing it in libmd I
can often see performance improvements:

For example with md5(1):
Current BUFSIZ (1024)
0.13 real         0.04 user         0.09 sys
New BUFSIZ (8192)
0.08 real         0.04 user         0.03 sys

sha256(1):
Before:
0.44 real         0.39 user         0.04 sys
After:
0.37 real         0.35 user         0.01 sys

This is done on a small amd64 Lenovo S20 laptop

Review available here:
https://reviews.freebsd.org/D2515

Any opinion?
Best regards,
Bapt