igrep.icn: Program for string search similar to egrep

Program to emulate UNIX egrep, but using the enhanced regular
expressions supported by regexp.icn.  Options supported are nearly
identical to those supported by egrep (no -b:  print disk block
number).  There is one additional option, -E, to allow Icon-type
(hence C-type) string escape sequences in the pattern string.
BEWARE:  when -E is used, backslashes that are meant to be processed
in the regular expression context must be doubled.  The following
patterns are equivalent:

 without -E: '\bFred\b'
 with    -E: '\\bFred\\b'

To enable the -D option (intended mainly for debugging), the Icon
Program Library file "ximage" must be linked with this program.