Load system's default trusted Certificate Authorities if available.

 src/gnutls.c (gnutls_certificate_set_x509_system_trust)
 [GNUTLS >= 3.0.20]: Declare for WINDOWSNT.
 (init_gnutls_functions)(gnutls_certificate_set_x509_system_trust)
 [GNUTLS >= 3.0.20]: Load from shared library for WINDOWSNT.
 (fn_gnutls_certificate_set_x509_system_trust) [!WINDOWSNT]: Define
 new macro.
 (Fgnutls_boot) [GNUTLS >= 3.0.20]: Call
 gnutls_certificate_set_x509_system_trust.  Log an error message if
 it fails.
Adjust lisp/ChangeLog for custom-prompt-customize-unsaved-options.

Reworded ChangeLog entry based on Stefan Monnier's suggestions.