Improve the usage of variable names in C docstrings.

Improve the usage of variable names in C docstrings.

* libguile/alist.c:
* libguile/array-map.c:
* libguile/arrays.c:
* libguile/bitvectors.c:
* libguile/filesys.c:
* libguile/foreign.c:
* libguile/generalized-arrays.c:
* libguile/hashtab.c:
* libguile/ioext.c:
* libguile/load.c:
* libguile/numbers.c:
* libguile/ports.c:
* libguile/posix.c:
* libguile/print.c:
* libguile/procprop.c:
* libguile/promises.c:
* libguile/simpos.c:
* libguile/socket.c:
* libguile/srfi-1.c:
* libguile/srfi-13.c:
* libguile/srfi-14.c:
* libguile/stacks.c:
* libguile/stime.c:
* libguile/strings.c:
* libguile/struct.c:
* libguile/symbols.c:
* libguile/threads.c:
* libguile/weak-table.c:
* libguile/weak-vector.c: Make the variable names in the C docstrings more
  consistent.  Replace a few instances of @var with @code when appropriate.