Allow and correctly handle unit prefix in the context of values

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Allow and correctly handle unit prefix in the context of values. For example, unit prefixes - k (kilo), M (mega), G (giga), T (tera).

And after clicking the Save button, I get:

Validation error for variable thold_warning_hi, with value of "65M". See backtrace below for more details. (/plugins/thold/thold.php: 64 save_thold)(/plugins/thold/thold_functions.php: 2980 get_filter_request_var)(/lib/html_utility.php: 423 die_html_input_error)(/lib/html_validate.php: 60 cacti_debug_backtrace)