now that we have a bundled libtls we can actually do this. Retain the knob to disable it "just in case".

gmid (like all other daemons that want to do privsep crypto) has a very close relationship with libtls and need to stay in sync with it. OpenBSD' libtls was recently changed to use OpenSSL' EC_KEY_METHOD instead of the older ECDSA_METHOD, on the gmid side we have to do the same otherwise failures happens at runtime. In a similar manner, privsep crypto is silently broken in the current libretls (next version should fix it.) The proper solution would be to complete the signer APIs so that applications don't need to dive into the library' internals, but that's a mid-term goal, for the immediate bundling the 'little' libtls is the lesser evil. The configure script has gained a new (undocumented for the time being) flag `--with-libtls=bundled|system' to control which libtls to use. It defaults to `bundled' except for OpenBSD where it uses the `system' one. Note that OpenBSD versions before 7.3 (inclusive) ought to use --with-libtls=bundled too since they still do ECDSA_METHOD.

Set a sane default for INSTALL, allow it to be changed either as environment variable or configure argument, and propagate it correctly to the generated config.mk. Issue reported by xavi, thanks!

also, while here, add some error checking too

replace its only usage with strncmp(). it's likely faster too.

We might end up calling client_close() from start_reply(), but that will free the fcgi/proxy bufferevent while they're still used on the stack. Instead, start_reply() only sets REQUEST_DONE and exits, returning the error eventually, so callers know when to stop.