Fix parsing bug in linux get_total_system_memory().

Nick Mathewsonrequested to merge
nickm/tor:bug40315_045 into maint-0.4.5

Use find_str_at_start_of_line(), not strstr() here: we don't want to match "MemTotal: " if it appears in the middle of a line.

Fixes #40315 (closed); bugfix on 0.2.5.4-alpha.

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