Fix bug when first byte from a LESSOPEN script is non-ASCII.

open_altfile reads one byte to see if the LESSSOPEN script will
produce any data, then pushes it back on the input queue via
ch_ungetchar. But the variable is declared as "char", so if the
first byte has the high bit set, and the compiler uses signed chars,
this will sign-extend and pass a negative value to ch_ungetchar.
ch_ungetchar takes a negative value as a signal to discard any
queued char.

filename.c

@@ -912,7 +912,7 @@ public char * open_altfile(constant char *filename, int *pf, void **pfd)
 #if HAVE_FILENO
 	if (returnfd)
 	{
-		char c;
+		unsigned char c;
 		int f;
 
 		/*