cygwin: mosh-executable-path should include last '/'
bignum mini-gmp: Fix ratnum div

mpq_div would have res == y so we should save res in tmp variable.

This patch fixes (/ 3 4 5) case.

FAILs:

 - (fac 4.5) => assertion
 - (denominator (inexact (/ 6 4))) => 2.0 (result 1.0)
   - (exact 1.5) => 3/2 (result 1, simplified case)
 - (rationalize (exact 0.3) 1/10)  => 1/3 (result 1/0)
   - (exact 0.3) => 5404319552844595/18014398509481984 (result 1/0)
bignum mini-gmp: Fix mpq_set_d

Obviously we need some bias here.. Use DBL_MANT_DIG bias for now.

Fixes:
 - (exact 1.5)
 - (exact 0.3)
 - (exact 3.0)

R6RS-test-suite result is now per with normal-gmp.