classic-ui: fix possible buffer overflows #5250

Add checks to functions using strcat. Now the legth of the resulting
string gets calculated and checked if it would exceed the max length
for this char array.

Thanks to DTAG Group Information Security for the findings.

refs: #5250
whatthecommit: fixed my stupidness ...
classic-ui: fix Off-by-one memory access in process_cgivars() #5251

Wrong increment in process_cgivars() got removed.

Thanks to DTAG Group Information Security for the findings.

refs: #5251
whatthecommit: Obligatory placeholder commit message
clssic-ui: fix status output in JSON format not including short and long plugin output properly #5217

A stupid logical mistake got fixed. Tested andworked here.

refs: #5217
whatthecommit: Fixed unnecessary bug.
Merge branch 'fix/json-long-status-output-5217' into support/1.10

Fixes: #5217
Merge branch 'fix/buffer-overflows-5250' into support/1.10

Conflicts:
	Changelog

Fixes: #5250
Merge branch 'fix/Off-by-one-memory-access-5251' into support/1.10

Conflicts:
	Changelog

Fixes: #5251