ifdef cleanup

Get rid of a lot of unnecessary ifdefs that just make the code harder to understand and aren't used on the platforms we support today.

• unifdef ultrix
• unifdef sony
• unifdef PowerMAX_OS
• unifdef __convex__
• unifdef hpux
• unifdef pegasus
• unifdef __UNIXOS2__
• unifdef QNX4
• unifdef AF_CHAOS
• unifdef AF_DECnet
• unifdef SYSV
• unifdef SIGNALS_RESET_WHEN_CAUGHT
• unifdef UNRELIABLE_SIGNALS
• unifdef FRAGILE_DEV_MEM
• Remove support for SVR4 systems other than Solaris/illumos
• include <shadow.h> on all platforms with HAVE_GETSPNAM set, not just SVR4
• unifdef __osf__
• session.c: clear -Wredundant-decls warnings on Solaris