Check the entire length of the current argument for the EOF string with -E,

but don't expect a proper ASCII string to exist right here right now, don't
use strcmp(3) which checks for a NUL.  As we're still building the argument
up, the next character might be garbage.  It would probably be just as safe to
temporarily write a NUL there, but if we've reached the end of argument memory
that might not be the best idea, I think.  It's unclear.

Doing it this way seems to meet the most with the original intent.

PR:		85696
Prodded by:	stefanf

usr.bin/xargs/xargs.c

@@ -304,7 +304,7 @@ arg1:		if (insingle || indouble)
 			errx(1, "unterminated quote");
 arg2:
 		foundeof = *eofstr != '\0' &&
-		    strcmp(argp, eofstr) == 0;
+		    strncmp(argp, eofstr, p - argp) == 0;
 
 		/* Do not make empty args unless they are quoted */
 		if ((argp != p || wasquoted) && !foundeof) {