Support .St -xsh4.2, the System Interfaces part of the original Single

Support .St -xsh4.2, the System Interfaces part of the original Single
UNIX Specification.  As this one appears to be used in the wild and we
already have -xpg4.2 and even -xsh5, it makes sense to add this one.
Note that calling the original SUS XPG4.2 appears to be more common
than calling it SUSv1, so it's ok that we don't have .St -susv1.
From Sascha Wildner <saw at online dot de> (DragonFly) via Franco Fichtner.