Reduced build warnings [-Wmissing-declarations]

[rousskov]

This change fixes a few manually picked cases in sources that are not
normally compiled on our CI nodes. There are probably more missing
declarations, but we do not know how to find them automatically.

Also removed a few seemingly unused functions and declarations.

[rousskov]

This is a draft PR because I cannot test these changes. This PR probably should not be merged without testing in environment(s) where the modified files are compiled.

The changes themselves were prompted by a "Can't build squid on Windows" email sent to Project info alias:

This is the version I have: "squid-6.9-20240523-rfb0d1814dd.tar.gz". 

I set up Cygwin environment on Windows and followed the instructions. Midway through the `make` command, I get errors, starting with the ones mentioned below: 

ext_lm_group_acl.cc:109:1: error: no previous declaration for ‘char* AllocStrFromLSAStr(LSA_UNICODE_STRING)’ [-Werror=missing-declarations]
  109 | AllocStrFromLSAStr(LSA_UNICODE_STRING LsaStr)
      | ^~~~~~~~~~~~~~~~~~
ext_lm_group_acl.cc:131:1: error: no previous declaration for ‘char* GetDomainName()’ [-Werror=missing-declarations]
  131 | GetDomainName(void)
      | ^~~~~~~~~~~~~
In file included from ../../../../compat/compat.h:104,
                 from ../../../../include/squid.h:32,
                 from ext_lm_group_acl.cc:78:

[yadij]

FTR; A cross-build of these helpers with MinGW64 confirms this PR to fix the errors mentioned by @rousskov. There are still many others to fix in them, but this is at least some progress.

[squidadm]

queued for backport to v6