util: use a shared lookup function for string tables

Macro DEFINE_STRING_TABLE_LOOKUP expands to a new function for each
of the almost 120 tables throghout the code.
Move the its implementation to a function (guaranteed to never be inlined),
and make the macro expand to an inlined function that calls this function.
This saves a few kilobytes from the systemd binary

(David: - fix coding-style
        - use 'ssize_t' to fix 32bit to 64bit propagation
        - use streq_ptr())