Remove unnecessary assignment by reference

From dhx's list, most items are in fact not pass-by-references at all.
The list returned by the following command excludes obvious false
positives, and the remaining occurences are not pass-by-references
either.

$ git grep '&\$' |grep -vE "function|Binary|@param"

Fixes #10976
Correct CSS property
Remove wrong phpdoc tag
Fix access of undefined variable

No runtime effect as this code branch is not used at the moment.
Remove useless code
Fall back to English when translated error string is missing

Fixes #16549
Make the error number a parameter in $s_missing_error_string
Use vsprintf instead of call_user_func_array( 'sprintf', ...)
travis: make travis_before_script fail on all errors
travis: use -e instead of -x
icon_api: added missing $t_status_icon_arr

Conflicts:
	library/adodb
	library/phpmailer
user_api: fix typos and incorrect argument count

It should be $t_last_timestamp_threshold not $c_last_timestamp_threshold
and although it probably would be a good idea to use a seed,
auth_generate_unique_cookie_string() doesn't actually take one.

Conflicts:
	core/user_api.php
	library/adodb
	library/phpmailer
Don't try to output result of print_user()

Conflicts:
	library/adodb
	library/phpmailer
Fix invocation of print_captcha_input() with too many arguments

Conflicts:
	library/adodb
	library/phpmailer
	signup_page.php
Fix invocation of plugin_pop_current() with wrong number of arguments

Conflicts:
	library/adodb
	library/phpmailer
Fix invocation of db_param() with incorrect number of arguments

Conflicts:
	core/version_api.php
	library/adodb
	library/phpmailer
Fix invocation of prepare_version_string() with incorrect number of arguments

Conflicts:
	bug_view_inc.php
	library/adodb
	library/phpmailer
Correct invocation of user_pref_get() with incorrect number of arguments

Conflicts:
	library/adodb
	library/phpmailer
Fix invocation of user_get_email() with incorrect number of arguments

Conflicts:
	account_page.php
	library/adodb
	library/phpmailer
Don't try to output return value of print_* functions

These functions don't return anything

Conflicts:
	adm_config_report.php
	library/adodb
	library/phpmailer
	manage_user_page.php