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

#ifndef DROPBEAR_DBHELPERS_H_
#define DROPBEAR_DBHELPERS_H_

/* This header defines some things that are also used by libtomcrypt/math. 
   We avoid including normal include.h since that can result in conflicting 
   definitions - only include config.h */
#include "config.h"

#ifdef __GNUC__
#define ATTRIB_PRINTF(fmt,args) __attribute__((format(printf, fmt, args))) 
#define ATTRIB_NORETURN __attribute__((noreturn))
#define ATTRIB_SENTINEL __attribute__((sentinel))
#else
#define ATTRIB_PRINTF(fmt,args)
#define ATTRIB_NORETURN
#define ATTRIB_SENTINEL
#endif

void m_burn(void* data, unsigned int len);

#endif /* DROPBEAR_DBHELPERS_H_ */