fix some gcc warnings (#73)

* tweak string size

fix gcc8 warnings

```
svr-agentfwd.c: In function 'bindagent':
svr-agentfwd.c:254:53: warning: '%s' directive output may be truncated writing up to 107 bytes into a region of size between 0 and 107 [-Wformat-truncation=]
  snprintf(addr.sun_path, sizeof(addr.sun_path), "%s/%s", path, sockfile);
                                                     ^~         ~~~~~~~~
svr-agentfwd.c:254:2: note: 'snprintf' output between 2 and 216 bytes into a destination of size 108
  snprintf(addr.sun_path, sizeof(addr.sun_path), "%s/%s", path, sockfile);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

* cleanup signed/unsigned comparison

fix gcc8 warnings
```
scp.c: In function 'do_local_cmd':
scp.c:132:17: warning: comparison of integer expressions of different signedness: 'u_int' {aka 'unsigned int'} and 'int' [-Wsign-compare]
   for (i = 0; i < a->num; i++)
                 ^
scpmisc.c: In function 'addargs':
scpmisc.c:161:25: warning: comparison of integer expressions of different signedness: 'int' and 'u_int' {aka 'unsigned int'} [-Wsign-compare]
  } else if (args->num+2 >= nalloc)
                         ^~
scpmisc.c: In function 'replacearg':
scpmisc.c:183:12: warning: comparison of integer expressions of different signedness: 'u_int' {aka 'unsigned int'} and 'int' [-Wsign-compare]
  if (which >= args->num)
            ^~
scpmisc.c: In function 'freeargs':
scpmisc.c:196:17: warning: comparison of integer expressions of different signedness: 'u_int' {aka 'unsigned int'} and 'int' [-Wsign-compare]
   for (i = 0; i < args->num; i++)
                 ^
```

see https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/misc.h.diff?r1=1.16&r2=1.17

1 files changed, 2 insertions, 2 deletions

diff --git a/scpmisc.h b/scpmisc.h
index f5e01d6..369b327 100644
--- a/scpmisc.h
+++ b/scpmisc.h
@@ -27,8 +27,8 @@ struct passwd *pwcopy(struct passwd *);
 typedef struct arglist arglist;
 struct arglist {
 	char    **list;
-	int     num;
-	int     nalloc;
+	u_int   num;
+	u_int   nalloc;
 };
 void	 addargs(arglist *, char *, ...);
 void	 replacearg(arglist *, u_int, char *, ...);