Use long when converting to/from time_t.

Fixes #33 (closed) and #32 (closed).

Replace scm_to/from_int with scm_to/from_long when dealing with time_t values.

To test this one can use a time value past 2038 in tests/x509-certificate-generation.scm for expiration/activation date. Before guile would error on converting the integer to int type. Not sure if this is something to test , there might be some architectures where long might still be to small to hold all time_t values.