Renamed/reverted file->buffer -> file->cache

And the related config options:

- cfg->file_buffer_size -> cfg->file_cache_size
- file->cfg->buffer_size -> file->cfg->cache_size
- file->cfg->buffer -> file->cfg->cache_buffer

The original motivation to rename this to file->buffer was to better
align with what other filesystems call this, but I think this is a case
where internal consistency is more important than external consistency.

file->cache better matches lfs->pcache and lfs->rcache, and makes it
easier to read code involving both file->cache and other user-provided
buffers.

Keeping the upstream name also helps with continuity.

tests/test_exhaustion.toml

@@ -242,8 +242,8 @@ defines.CKPROGS = 'BADBLOCK_BEHAVIOR >= LFS_EMUBD_BADBLOCK_READERROR'
 defines.N = [1, 2, 4, 8, 16, 32, 64]
 defines.SIZE = [
     '0',
-    'FILE_BUFFER_SIZE/2',
-    '2*FILE_BUFFER_SIZE',
+    'FILE_CACHE_SIZE/2',
+    '2*FILE_CACHE_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',
@@ -521,8 +521,8 @@ defines.BADBLOCK_BEHAVIOR = [
 # we need prog checking to detect read errors
 defines.CKPROGS = 'BADBLOCK_BEHAVIOR >= LFS_EMUBD_BADBLOCK_READERROR'
 defines.SIZE = [
-    'FILE_BUFFER_SIZE/2',
-    '2*FILE_BUFFER_SIZE',
+    'FILE_CACHE_SIZE/2',
+    '2*FILE_CACHE_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',
@@ -710,8 +710,8 @@ defines.CKPROGS = 'BADBLOCK_BEHAVIOR >= LFS_EMUBD_BADBLOCK_READERROR'
 defines.N = [1, 2, 4, 8, 16, 32, 64]
 defines.SIZE = [
     '0',
-    'FILE_BUFFER_SIZE/2',
-    '2*FILE_BUFFER_SIZE',
+    'FILE_CACHE_SIZE/2',
+    '2*FILE_CACHE_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',
@@ -1132,8 +1132,8 @@ defines.CKPROGS = 'BADBLOCK_BEHAVIOR >= LFS_EMUBD_BADBLOCK_READERROR'
 defines.N = [1, 2, 4, 8, 16, 32, 64]
 defines.SIZE = [
     '0',
-    'FILE_BUFFER_SIZE/2',
-    '2*FILE_BUFFER_SIZE',
+    'FILE_CACHE_SIZE/2',
+    '2*FILE_CACHE_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',