gensym thread safety

[Kevin Ryde]

The chance of two threads hitting the read+increment at the same time
is obviously small, but a mutex will guarantee it.

        * symbols.c (scm_gensym): Use scm_i_misc_mutex around gensym_counter
        update, for thread safety.
        (gensym_counter): Move into scm_gensym which is its only user.
        (scm_init_symbols): No need to explicitly initialize gensym_counter.

symbols.c.gensym.diff
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