Clean up expand.c

Clean up expand.c

Clarify the naming and documentation on functions in src/expand.c:
- variable_expand -> expand_string
- variable_expand_string -> expand_string_buf
- variable_expand_for_file -> expand_string_for_file
- allocated_variable_expand -> allocated_expand_string
- allocated_variable_expand_for_file ->
  allocated_expand_string_for_file
Change all callers to use the new names.

* src/variable.h: Rename the functions and macros.
* src/expand.c: Ditto.
* src/file.c: Use the new function names.
* src/function.c: Ditto.
* src/implicit.c: Ditto.
* src/job.c: Ditto.
* src/loadapi.c: Ditto.
* src/main.c: Ditto.
* src/read.c: Ditto.
* src/remake.c: Ditto.
* src/variable.c: Ditto.
* src/vpath.c: Ditto.
* src/w32/subproc/sub_proc.c: Ditto.