revert #652

reverting broad cosmetic/style change because:
- increases merge-conflicts
- increases overhead of merging upstream Vim patches
- reasons for change are ambiguous, so default to no change
Replace 'alloc' with 'xmalloc' in some files.

Files changed: charset.c, buffer.c, diff.c, edit.c,
ex_cmds.c, ex_cmds2.c and ex_docmd.c.

The remaining alloc's in these files require more careful attention to
remove.
Added missing #define guards in headers
replace some mch_stat() with os_file_exists()
use os_getperm in f_getfperm
rename buf_store_time to buf_store_file_info
implement os_file_get_size
implement FileInfo struct
replace some mch_stat with os_get_file_info