os: update comments

[ttytm]

Adds line breaks for comments that should define sections in the source code to not make them part of doc comments.

E.g., like it's already done here:

v/vlib/os/file.c.v

Lines 415 to 419 in 399af67

	// **************************** Read ops ***************************
	// fread wraps C.fread and handles error and end-of-file detection.
	fn fread(ptr voidptr, item_size int, items int, stream &C.FILE) !int {
	nbytes := int(C.fread(ptr, item_size, items, stream))

Removes one sectional comments for OS ops since all operations are OS ops in that file