doas: adjust yyerror() output

[Gleydson Soares]

I just stumbled over this...

% doas abc
syntax error at line 1
% 

I took some secs trying to figure out what was wrong with abc's command syntax 
that I typed out. 
But bingo,  It was happenned due my doas.conf has a syntax error...  
Seems that yyerror() doesn't print out the progname's string, 

sounds better for a quick diagnosis?

% doas abc
doas: syntax error at line 1
%

sounds better for a quick diagnosis?
OK?

Index: parse.y
===================================================================
RCS file: /cvs/src/usr.bin/doas/parse.y,v
retrieving revision 1.14
diff -u -p -r1.14 parse.y
--- parse.y     4 Dec 2015 09:41:49 -0000       1.14
+++ parse.y     26 Apr 2016 01:37:57 -0000
@@ -176,6 +176,7 @@ yyerror(const char *fmt, ...)
 {
        va_list va;
 
+       fprintf(stderr, "%s: ", getprogname());
        va_start(va, fmt);
        vfprintf(stderr, fmt, va);
        va_end(va);