New port: textproc/chkascii: Check an ASCII text file for invalid characters

chkascii is a small C program that checks files for any ASCII values
that are possibly junk. The following are considered the only good ASCII
values permissible in a text file: 32-126, 9, 10

The user can optionally force additional ASCII values to be treated as
good.

For good text files, chkascii will wind up by checking on proper EOL
termination. If the file is not EOL-terminated, that too is reported as
an error.

WWW: https://github.com/bourne-again/chkascii

PR: 233254
Submitted by: bourne.identity@hotmail.com
Reviewed by: krion
Approved by: krion (mentor)
Differential Revision: https://reviews.freebsd.org/D18047