example, tests: fix/silence -Wformat-truncation=2 gcc warnings

Then sync this warning option with curl.

Seems like a false positive and/or couldn't figure how to fix it, so silence:
```
example/ssh2.c:227:38: error: '%s' directive output may be truncated writing likely 1 or more bytes into a region of size 0 [-Werror=format-truncation=]
  227 |             snprintf(fn1, fn1sz, "%s/%s", h, pubkey);
      |                                      ^~
example/ssh2.c:227:34: note: assuming directive output of 1 byte
  227 |             snprintf(fn1, fn1sz, "%s/%s", h, pubkey);
      |                                  ^~~~~~~
example/ssh2.c:227:13: note: 'snprintf' output 3 or more bytes (assuming 4) into a destination of size 2
  227 |             snprintf(fn1, fn1sz, "%s/%s", h, pubkey);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
example/ssh2.c:228:38: error: '%s' directive output may be truncated writing likely 1 or more bytes into a region of size 0 [-Werror=format-truncation=]
  228 |             snprintf(fn2, fn2sz, "%s/%s", h, privkey);
      |                                      ^~
example/ssh2.c:228:34: note: assuming directive output of 1 byte
  228 |             snprintf(fn2, fn2sz, "%s/%s", h, privkey);
      |                                  ^~~~~~~
example/ssh2.c:228:13: note: 'snprintf' output 3 or more bytes (assuming 4) into a destination of size 2
  228 |             snprintf(fn2, fn2sz, "%s/%s", h, privkey);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
Ref: https://github.com/libssh2/libssh2/actions/runs/7055480458/job/19205970397#step:10:98

Fix:
```
tests/openssh_fixture.c:116:38: error: ' 2>&1' directive output may be truncated writing 5 bytes into a region of size between 1 and 1024 [-Werror=format-truncation=]
tests/openssh_fixture.c:116:11: note: 'snprintf' output between 6 and 1029 bytes into a destination of size 1024
```
Ref: https://github.com/libssh2/libssh2/actions/runs/7055480458/job/19205969221#step:10:51

Tested via #1257

acinclude.m4

@@ -499,7 +499,7 @@ AC_DEFUN([CURL_CC_DEBUG_OPTS],
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [restrict])
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [alloc-zero])
             tmp_CFLAGS="$tmp_CFLAGS -Wformat-overflow=2"
-            tmp_CFLAGS="$tmp_CFLAGS -Wformat-truncation=1"  # =2 causes false positives
+            tmp_CFLAGS="$tmp_CFLAGS -Wformat-truncation=2"
           fi
           #
           dnl Only gcc 10 or later

cmake/PickyWarnings.cmake

@@ -193,7 +193,7 @@ elseif(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX OR CMAKE_C_COMPILER_I
           -Walloc-zero                     #             gcc  7.0
           -Wduplicated-branches            #             gcc  7.0
           -Wformat-overflow=2              #             gcc  7.0
-          -Wformat-truncation=1            #             gcc  7.0 (=2 causes false positives)
+          -Wformat-truncation=2            #             gcc  7.0
           -Wrestrict                       #             gcc  7.0
         )
       endif()

example/ssh2.c

@@ -222,10 +222,18 @@ int main(int argc, char *argv[])
                 fprintf(stderr, "out of memory\n");
                 goto shutdown;
             }
+            /* Avoid false positives */
+#if defined(__GNUC__) && __GNUC__ >= 7
+#pragma GCC diagnostic push
+#pragma GCC diagnostic warning "-Wformat-truncation=1"
+#endif
             /* Using asprintf() here would be much cleaner,
                but less portable */
             snprintf(fn1, fn1sz, "%s/%s", h, pubkey);
             snprintf(fn2, fn2sz, "%s/%s", h, privkey);
+#if defined(__GNUC__) && __GNUC__ >= 7
+#pragma GCC diagnostic pop
+#endif
 
             if(libssh2_userauth_publickey_fromfile(session, username,
                                                    fn1, fn2,

tests/openssh_fixture.c

@@ -85,7 +85,7 @@ static int run_command_varg(char **output, const char *command, va_list args)
 
     FILE *pipe;
     char command_buf[BUFSIZ];
-    char buf[BUFSIZ];
+    char buf[BUFSIZ + sizeof(redirect_stderr)];
     int ret;
     size_t buf_len;