Eliminate 'cast between incompatible function types' compiler warning

Eliminate 'cast between incompatible function types' compiler warning

* cord/cordxtra.c (refill_cache): Add GC_CALLBACK; change return type
from char to void*; change argument type from refill_data* to void*;
add necessary casts to client_data and the return expression.
* cord/cordxtra.c (CORD_lf_func): Do not cast refill_cache.
* os_dep.c [MPROTECT_VDB] (GC_write_fault_handler,
GC_mprotect_dirty_init): Cast SIG_DFL, SIG_IGN and oldact.sa_handler
to SIG_HNDLR_PTR via signed_word type.
* os_dep.c [MPROTECT_VDB && !MSWIN32 && !MSWINCE]
(GC_write_fault_handler): Cast old_handler to PLAIN_HNDLR_PTR via
signed_word type.