Fix for MySQL 5.7+

https://github.com/friendica/friendica/issues/2786

Only tested on Ubuntu 16.10 with MySQL 5.7.17.

https://dev.mysql.com/doc/refman/8.0/en/timestamp-initialization.html

https://dev.mysql.com/doc/refman/8.0/en/sql-mode.html#sqlmode_no_zero_date
As of MySQL 5.7.4, NO_ZERO_DATE is deprecated.

Patch based on the following search & replace (and then reverting some
changes in libraries):

```sh
grep -l '= "0000-00-00"' -r . | xargs sed -i -e "s/= \"0000-00-00\"/=
'0000-00-00'/g"

grep -l "0000-00-00" -r . | xargs sed -i -e "s/0000-00-00/0001-01-01/g"

grep -l "!= '0001-01-01'" -r . | xargs sed -i -e "s/!= '0001-01-01'/>
'0001-01-01'/g"

grep -l " === '0001-01-01'" -r . | xargs sed -i -e "s/ === '0001-01-01'/
<= '0001-01-01'/g"

grep -l " == '0001-01-01'" -r . | xargs sed -i -e "s/ == '0001-01-01'/
<= '0001-01-01'/g"

grep -l "strpos(\$dob, '0000-') === 0" -r . | xargs sed -i -e
"s/strpos(\$dob, '0000-') === 0/strpos(\$dob, '0000-') === 0 ||
strpos(\$dob, '0001-') === 0/g"

grep -l 'array("", "0001-01-01")' -r . | xargs sed -i -e 's/array("",
"0001-01-01")/array("", "0000-00-00", "0001-01-01")/g'

grep -l "\$dob = '0000-'" -r . | xargs sed -i -e "s/\$dob =
'0000-'/\$dob = '0001-'/g"
```

mod/hovercard.php

@@ -75,7 +75,7 @@ function hovercard_content() {
 		'tags' => $contact["keywords"],
 //		'nsfw' => intval($contact["nsfw"]),
 //		'server_url' => $contact["server_url"],
-		'bd' => (($contact["birthday"] == "0000-00-00") ? "" : $contact["birthday"]),
+		'bd' => (($contact["birthday"] <= '0001-01-01') ? "" : $contact["birthday"]),
 //		'generation' => $contact["generation"],
 		'account_type' => account_type($contact),
 		'actions' => $actions,