common/mischelp: use platform memory zeroing function for wipememory

* common/mischelp.h (wipememory): Replace macro with function
prototype.
(wipememory2): Remove.
* common/mischelp.c (wipememory): New.
* configure.ac (AC_CHECK_FUNCS): Check for 'explicit_bzero'.
--

In new wipememory function, memory is cleared through platform
provided secure memory zeroing function, SecureZeroMemory
or explicit_bzero.

If none of these is available, memset is called through
volatile function pointer to so that compiler won't optimize
away the call.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>

configure.ac

@@ -1400,16 +1400,16 @@ AC_FUNC_FSEEKO
 AC_FUNC_VPRINTF
 AC_FUNC_FORK
 AC_CHECK_FUNCS([atexit canonicalize_file_name clock_gettime ctermid  \
-                fcntl flockfile fsync ftello ftruncate funlockfile   \
-                getaddrinfo getenv getpagesize getpwnam getpwuid     \
-                getrlimit getrusage gettimeofday gmtime_r            \
-                inet_ntop inet_pton isascii lstat                    \
-                memicmp memmove memrchr mmap nl_langinfo pipe        \
-                raise rand setenv setlocale setrlimit sigaction      \
-                sigprocmask stat stpcpy strcasecmp strerror strftime \
-                stricmp strlwr strncasecmp strpbrk strsep            \
-                strtol strtoul strtoull tcgetattr timegm times       \
-                ttyname unsetenv wait4 waitpid ])
+                explicit_bzero fcntl flockfile fsync ftello          \
+                ftruncate funlockfile getaddrinfo getenv getpagesize \
+                getpwnam getpwuid getrlimit getrusage gettimeofday   \
+                gmtime_r inet_ntop inet_pton isascii lstat memicmp   \
+                memmove memrchr mmap nl_langinfo pipe raise rand     \
+                setenv setlocale setrlimit sigaction sigprocmask     \
+                stat stpcpy strcasecmp strerror strftime stricmp     \
+                strlwr strncasecmp strpbrk strsep strtol strtoul     \
+                strtoull tcgetattr timegm times ttyname unsetenv     \
+                wait4 waitpid ])
 
 # On some systems (e.g. Solaris) nanosleep requires linking to librl.
 # Given that we use nanosleep only as an optimization over a select