mk: Rewrite the checksum script in awk.

The previous shell script version's runtime was quadratic against the
number of distfiles to verify.  Historically this has not been an issue,
with usually only a handful of files per package.  However, with the
introduction of Go modules the number of distfiles used by a single
package can be very high.

For example, in an upcoming update of www/grafana to version 7.1.5, the
number of GO_MODULE_FILES is 821.  Running 'bmake checksum' takes:

  real    18m20.743s
  user    17m27.975s
  sys    0m49.239s

With the awk code, this is reduced to a far more sensible:

  real    0m4.330s
  user    0m3.241s
  sys    0m0.875s

The script has been written to emulate the previous version precisely,
preserving the same output and error messages and supporting all of its
behaviour, with the one exception that previous exit values of 128 have
been changed to 3, in order to avoid any potential signed 8-bit issues.

The one change in the pkgsrc infrastructure is that the mk/fetch/fetch
script no longer sets a working default value for ${CHECKSUM}.  This is
not a problem in a pkgsrc environment as all of the required variables
are set correctly, but if there happen to be any users who are using
this script in a standalone environment, they will need to set it
accordingly.  This was probably required in many situations previously
anyway, as none of the script's environment variables were set, and
trying to support this would be fragile at best.

(jperkin)