* 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
