use strlcpy & strlcat (#74)

* refactor checkpubkeyperms() with safe BSD functions

fix gcc8 warnings
```
svr-authpubkey.c: In function 'checkpubkeyperms':
svr-authpubkey.c:427:2: warning: 'strncat' specified bound 5 equals source length [-Wstringop-overflow=]
  strncat(filename, "/.ssh", 5); /* strlen("/.ssh") == 5 */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
svr-authpubkey.c:433:2: warning: 'strncat' specified bound 16 equals source length [-Wstringop-overflow=]
  strncat(filename, "/authorized_keys", 16);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

see https://www.sudo.ws/todd/papers/strlcpy.html

* restore strlcpy in xstrdup

see original https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/xmalloc.c?rev=1.16

1 files changed, 1 insertions, 1 deletions

diff --git a/scpmisc.c b/scpmisc.c
index 33e1891..c2f053e 100644
--- a/scpmisc.c
+++ b/scpmisc.c
@@ -102,7 +102,7 @@ xstrdup(const char *str)
 
 	len = strlen(str) + 1;
 	cp = xmalloc(len);
-	strncpy(cp, str, len);
+	strlcpy(cp, str, len);
 	return cp;
 }