curlx: limit use of system allocators to the minimum possible

Clone a multibye conversion function into curlx/fopen, and use that
local copy from curlx/fopen functions. Adjust allocators in curlx/fopen
to use curl's in normal builds, and system allocators in TrackMemory
builds to avoid recursion.

This allows to switch curlx/multibyte functions to curl allocators in
all configurations, as they are no longer called by curlx/fopen, and
a recursive call can no longer happen.

After this patch the system allocator is only used in TrackMemory
Windows builds, within curlx `fopen`, `freopen`, `stat` and `open`
functions.

Also:
- test 1, 440, 767: raise allocation limitsto fit the extra allocations
  in Windows Unicode builds.
- replace all uses of `curlx_unicodefree()` macro with `curlx_free()`
  across the codebase.
- curlx/multibyte: delete `curlx_unicodefree()`.
- ldap: join Windows and non-Windows codepaths that became
  identical after moving from `curlx_unicodefree()` to `curlx_free()`.
- vauth: drop a strdup from standard to curl allocator since
  the original allocation is now already done by curl's.
- tool_doswin: drop now superfluous strdup from `FindWin32CACert()`.
- memanalyzer.pm: sync weirdo `calloc` log message with `malloc`'s.

Fixes #19748
Closes #19845

tests/data/test1

@@ -50,7 +50,7 @@ Accept: */*
 
 </protocol>
 <limits>
-Allocations: 120
+Allocations: 135
 Maximum allocated: 136000
 </limits>
 </verify>

tests/data/test440

@@ -74,7 +74,7 @@ https://this.hsts.example./%TESTNUMBER
 56
 </errorcode>
 <limits>
-Allocations: 145
+Allocations: 160
 </limits>
 </verify>
 </testcase>

tests/data/test767

@@ -51,7 +51,7 @@ Accept: */*
 
 </protocol>
 <limits>
-Allocations: 120
+Allocations: 135
 Maximum allocated: 136000
 </limits>
 </verify>