Include <dirent.h> instead of <sys/dirent.h> for POSIX compliancy.

Fixes build errors on Linux, tested on CentOS.
The problem was pointed out by Valery Ushakov.
Note <sys/param.h> will be prepared by src/tools/compat/configure.

XXX: not sure if we still need this dumb tool for a TierII port...
fix size in memset() parameter. Thanks max.
add bio(4) support
xref mpt(4)
xref bio(4)
mpt(4): added bio(4) support.
Add execvpe in one more place to explain its last argument.
Call character '0' 'NUL' instead of 'null'.
show "sd0" instead of "mpt0" for volume device name
include vendor/product/rev info for volumes and their disks
no longer needed
implement BIOCDISK_NOVOL