illumos gate

Build errors on SPARC:

    common/modem_setup.c: In function 'ADM_Process_modem_setup':
    common/modem_setup.c:99:6: error: cast between incompatible function types from 'void (*)()' to 'void * (*)(void *)' [-Werror=cast-function-type]
       99 |      (void * (*)(void *))ADM_Modem_Listen, (void *)NULL) != 0) {
          |      ^
    cc1: all warnings being treated as errors
    *** Error code 1
    dmake: Fatal error: Command failed for target `modem_setup.o'
    Current working directory /code/illumos-gate/usr/src/cmd/scadm/sparc/mpxu

Fix function signature.

Testing done: build/install/boot.