build: sync up picky warnings with curl, fix fallouts

- cmake: sync `-ftree-vrp` behavior with autotools.
- build: enable `-Wjump-misses-init` for GCC 4.5+.
  Credits-to: Marcel Raad
- packet: fix `-Wjump-misses-init` warnings.
  ```
  src/packet.c: In function ‘_libssh2_packet_add’:
  src/packet.c:671:9: error: jump skips variable initialization [-Werror=jump-misses-init]
  src/packet.c:920:31: note: ‘want_reply’ declared here
  src/packet.c:671:9: error: jump skips variable initialization [-Werror=jump-misses-init]
  src/packet.c:919:26: note: ‘len’ declared here
  src/packet.c:669:9: error: jump skips variable initialization [-Werror=jump-misses-init]
  src/packet.c:1121:31: note: ‘want_reply’ declared here
  src/packet.c:669:9: error: jump skips variable initialization [-Werror=jump-misses-init]
  src/packet.c:1120:26: note: ‘len’ declared here
  src/packet.c:669:9: error: jump skips variable initialization [-Werror=jump-misses-init]
  src/packet.c:1119:26: note: ‘channel’ declared here
  ```
- build: enable gcc-12/13+, clang-10+ picky warnings
- acinclude.m4: sync formatting/comments with curl.
- autotools: fix `-Wtrampolines` picky warning for gcc 4.x versions.
  Follow-up to 854cfa8292 #1524
- cmake: enable `-Wall` for MSVC when `PICKY_COMPILER=ON`.
- MSVC: fix `-Wall` warnings.
  Seen on VS2015. Not seen on VS2022. Unknown for other versions.
  ```
  tests\test_simple.c(60): warning C4777: 'fprintf' : format string '%d' requires an argument of type 'int', but variadic argument 1 has type 'std::size_t'
  tests\test_simple.c(60): warning C4777: 'fprintf' : format string '%.*s' requires an argument of type 'int', but variadic argument 2 has type 'std::size_t'
  ```
- mbedtls: stop silencing warnings in 3rd-party header.
  Follow-up to a3aa6b4ca8 #1525
- cmake: stop deleting `-W<n>` from `CMAKE_C_FLAGS` (MSVC)
  1. `CMAKE_C_FLAGS` may apply to other projects, and deleting/altering it
     may be unexpected.
  2. We pass `-W4`/`-Wall` internally now, which do override custom
     `-W<n>` options as tested with VS2008 and newer VS generators.

Closes #1588

tests/test_simple.c

@@ -58,7 +58,7 @@ static int test_libssh2_base64_decode(LIBSSH2_SESSION *session)
     if(datalen != 5 || strcmp(data, "fnord") != 0) {
         fprintf(stderr,
                 "_libssh2_base64_decode() failed (%d, %.*s)\n",
-                (unsigned int)datalen, (unsigned int)datalen, data);
+                (int)datalen, (int)datalen, data);
         return 1;
     }