WebHTTrack keeps option fields it never sends, and mutes a blank User-Agent

[xroche]

The wizard's type-check list and its eight extension/MIME pairs were written to winprofile.ini and read back into the form, but step4.html never put them on the command line, so the crawl never saw them. The browser-id box had the opposite problem: cleared, it emitted --user-agent "", and an empty -F switches the header off instead of falling back to the engine's own value. The command block now emits --check-type=N, one --assume "ext=mime" per filled pair, and nothing at all for a blank browser id. This is engine-side only: the keys already exist in the shared winprofile block, hts_optalias already carries check-type and assume, and winprofile-keys.tsv is untouched.

The second commit fixes a footgun the first one opened and a twin already on master. ${arg:mimeN} sat outside the ${test:extN:} guard, so emptying an extension while leaving its MIME type filled dropped a bare token onto the command line, which the engine reads as options of its own: a MIME field holding -O /some/path silently relocates the whole mirror. ${arg:portprox} has the same shape and predates this PR. Nine sites in all, now wrapped in ${do:if-not-empty:<guard>} the way the proxy scheme already was. htsserver is a local UI and the person filling the form is the person whose crawl it is, so this is a way to lose work rather than a privilege boundary anyone crosses, but emptying one box and not its neighbour is an ordinary thing to do. Test 358 asserted the absence of =<mime> rather than of the value, so it went green on the injecting tree; it now asserts the value is gone and probes the shape directly.

windebug is the original bug mirrored, reaching the engine as --debug-headers and never saved, so it reverts on reload. That one needs a new key in the shared block and WinHTTrack's own spelling has to decide it, so I left it to the httrack-windows side. An extension beginning with a dash still kills the run before any log is written, which is #1179's narrow allowlist rather than anything here: filed as #1425. Test 274 asserted --user-agent "" as the observable for #1186, that a key present in the file beats the wizard's startup default; the property still holds and is now checked on the rendered box, which is what #1186 was about.

Verified against a running htsserver rather than by reading templates: the new test posts browser-shaped forms, starts a real crawl and reads hts-cache/doit.log for the argv the engine got. The --assume half also has to change the mirror, since data.foo is only retyped and parsed once the rule arrives.

Closes #1386