Install: always report a warning for a blank admin password

Use the DB password as admin password
Install: Move DB hostname check after DB type

This is more logical and improves code readability
Install: regroup the form variables initialization
Install: Code cleanup, remove useless if statements

It's not necessary to individually test the same variable to
conditionally display install-only fields
Install: fix problem with db_password when retrying after failure

The logic to avoid divulgation of db password was flawed, forcing user
to re-enter the password when retrying after a failed install/upgrade.
Install: fix logic to retry a failed install

Instead of having a single form spanning the entire page, we have one
to display input fields for user to submit new or modified data, and one
with hidden fields to retry failed steps.

With this, the retry button now works as expected.
Install: protect admin password

The admin password must be protected in the same way as the db password,
to avoid divulgating it.
Install: allow admin to select time zone

Fixes #15205
Install: remove indentation in generated config_inc.php file

Also align the '=' signs
Install: allow admin to specify the cryptographic salt

A strong master salt is automatically generated if possible, otherwise
the user will have to provide this value manually.
Added comments, whitespace fixes

Moved the "print sql queries" to the end of the form, just before the
submit button
Install: Removed invalid </input> end tags

<input> is a void tag, using </input> is invalid HTML syntax [1]

[1] http://www.w3.org/TR/html-markup/syntax.html#void-element
Install: use standard html API to generate head
Install: allow admin to specify table pre/suffix

Fields are predefaulted to config_default_inc.php values, adequate
Oracle-compatible short values are set when db_type is oci8.

A jQuery script takes care of automatically updating the input fields
contents based on the picked value in db_type selection list. It works
by picking up the appropriate default values from hidden divs.
Install: perform DB version check while connected as admin

This avoids the issue where the call to ServerInfo() only works on MySQL
if the database is open, which may not be the case when testing with
database user (e.g. if user does not exist or does not have access)

Fixes #7635
Various fixes and improvements to the installer

Fixes #9701 #16357 #7632 (and potentially others as well)
Install: added ADOdb library version check