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, 1 insertions, 1 deletions

diff --git a/svr-agentfwd.c b/svr-agentfwd.c
index 37c30b0..cff80f7 100644
--- a/svr-agentfwd.c
+++ b/svr-agentfwd.c
@@ -209,7 +209,7 @@ static int bindagent(int fd, struct ChanSess * chansess) {
 
 	struct sockaddr_un addr;
 	unsigned int prefix;
-	char path[sizeof(addr.sun_path)], sockfile[sizeof(addr.sun_path)];
+	char path[(sizeof(addr.sun_path)-1)/2], sockfile[(sizeof(addr.sun_path)-1)/2];
 	mode_t mode;
 	int i;
 	uid_t uid;