Refactored SYSTEMD_CGROUP_DEV

[cavaliercoder]

I've made a few stylistic changes to SYSTEMD_CGROUP_DEV for your review @jangaraj.

• removed all malloc calls to prevent dangling pointers - there were a few returns that didn't free allocated pointers
• removed log messages that should be logged already by SET_MSG_RESULT
• simplified string concats using zbx_snprintf
• sanitised request parameters
• moved variable declarations to the the start of the function
• memoized string lengths to prevent multiple calls to strlen
• search for \0 at end of string instead of appending \0 to a string and then calling strncmp

if you're happy with these, please merge the PR.

[jangaraj]

It looks good to me. Thank you Ryan.