filesystem: always report the filename extension

[msetaro]

Unix and Windows Sys_LoadLibrary print FS_AllowedExtension's output on a true return, but the function only populated that output on false returns. Initialize the supplied output for every filename, using an empty string when there is no dot. Preserve classification, casing, and the special .so.N suffix.

This fixes the shared output contract for both platform callers. Existing callers that inspect the output only on false returns keep the same strings.

Regression:

gcc -O2 -fno-strict-aliasing -ffunction-sections -fdata-sections \
  tools/tests/extension_output.c code/qcommon/files.c code/qcommon/q_shared.c \
  -Wl,--gc-sections -o /tmp/extension-output
/tmp/extension-output

Before: the output assertion fails for an extensionless name. After: empty/extensionless/trailing-dot, ordinary extension, versioned .so and allowed/disallowed pk3 cases pass. The separate pointer-arithmetic fix in #425 is unchanged by this patch.

The fork includes these assertions in its existing unit driver. Sanitizer/pointer modes, original unit/collision/smoke/frame goldens pass. Production symbol gate passes; machine-code bytes change only in FS_AllowedExtension among 99 function sections, for the added output initialization. The text diff also renames a compiler switch-table label in FS_Seek; its bytes are unchanged. Tracked in msetaro#31.