build: Merge version.{c,h} into string.{c,h}.

build: Merge version.{c,h} into string.{c,h}.

This is a preparatory patch for the upcoming revamp of the git
version script. The revamped script generates version.c, a file
which is currently tracked by git. We can easily get rid of it (and
of version.h) since all paraslash executables link in both string.o
and version.o. So let's move the three small functions of version.c
to string.c and their declarations from version.h to string.h.