nuke strncpy

[Jim Meyering]

I noticed that there were some unnecessary uses of strncpy in bison.
The first patch removes them.
Along the way I noticed a 3-line xmalloc+memcpy+zero-one-byte sequence
that can be replaced by a single-line xstrdup use.  That's #2.
And #3 merely regenerates parse-gram.[ch].

[PATCH 1/3] maint: s/strncpy/memcpy/, when equivalent
[PATCH 2/3] maint: simplify parse-gram.y
[PATCH 3/3] maint: regen src/parse-gram.[ch]